Planning Tools · 19 min read
3 Free Home Design Calculators: Paint, Rug Size & Room Lighting
Donald Martin
Good rooms are not lucky. Behind nearly every space that feels right — the rug that grounds the sofa instead of floating under the coffee table, the walls that got two even coats from a single trip to the store, the light that flatters instead of glares — there is a small amount of arithmetic that happened before anyone picked up a paintbrush.
That math is not complicated. It is just easy to skip, and skipping it is what sends people back to the store for a third gallon, or leaves them with a 5×7 rug marooned in a 14-foot room. So I built the three calculators on this page to do the measuring for you. They are free, need no sign-up, and run entirely in your browser — nothing you type is sent anywhere.
There is one more thing I insisted on: every calculator shows its own formula and updates it with your numbers as you type. You should never have to trust a black box that spits out "buy 4 gallons" with no explanation. Watch the math change, and you will understand your own room better — which is the whole point. Let's start with the one that saves the most money and the most second trips: paint.
1. The Paint Calculator
≈20 sq ft each
≈15 sq ft each
Or 3 gallons if your walls are textured, porous or going dark-to-light (350 sq ft per gallon instead of 400).
gallons = ⌈ (2·(L+W)·H − 20·doors − 15·windows) × coats ÷ 400 ⌉ = ⌈ (2·(12+14)·8 − 20·1 − 15·2) × 2 ÷ 400 ⌉
= ⌈ 366 × 2 ÷ 400 ⌉ = ⌈ 1.83 ⌉ = 2 galOne gallon covers about 400 sq ft per coat at best; a door removes ~20 sq ft and a window ~15 sq ft of paintable wall. Gallons are rounded up so you keep a touch-up margin.
Estimate A planning figure, not a purchase order. Confirm the coverage printed on the can of the exact product you buy, and add a gallon if the walls are textured, porous or bare drywall.
Run a tape around the room, multiply that perimeter by the ceiling height, subtract 20 sq ft for every door and 15 for every window, multiply what is left by the number of coats, divide by the square footage one gallon covers and round up to the next whole can. If you are painting the ceiling too, its area is figured on its own at one coat and rounded up on its own, because it is a separate can of a separate product.
What it assumes
- One US gallon covers 400 sq ft per coat — the top of the 350–400 sq ft range that Sherwin-Williams and This Old House both quote. Benjamin Moore rates Regal Select at 400–450; some Behr lines run as low as 250. Textured, porous or bare-drywall walls, deep saturated colours and gloss sheens all drink more, so treat 400 as the best case and buy the extra gallon when any of those apply.
- The room is treated as a rectangular box: wall area = perimeter × ceiling height, ceiling area = length × width. Alcoves, soffits, bay windows and sloped ceilings are not modelled — split an irregular room into rectangles and add the results.
- Each door removes 20 sq ft and each standard window 15 sq ft of paintable wall. Guides differ: This Old House uses 10 sq ft per window, while Behr and Home Improvement For Dummies use 15. We use 15, the more conservative deduction. If your openings are unusually large, measure them.
- Two coats is the default because that is what Benjamin Moore's own calculator assumes and what This Old House calls the rule of thumb. Use one coat only when refreshing the same colour, three when a pale colour goes over something dark.
- The ceiling is figured at one coat and its gallons are rounded up separately, then added, because ceiling paint is a different product in a different can — leftover wall paint cannot cover the ceiling's last half gallon.
- Gallons round up to the next whole can, so the figure includes a touch-up margin and can over-buy by up to most of a gallon on a job that lands just over a whole number.
- This is wall and ceiling paint only. Primer covers far less — Sherwin-Williams puts it at 200–300 sq ft per gallon — and trim, baseboards, casings and the doors themselves are separate products and a separate calculation, even though their area has been deducted here.
Sources
- Paint Calculator: How Much Paint Do I Need? — This Old House (350–400 sq ft per gallon and uses ~400; 20 sq ft per door, 10 sq ft per window; perimeter × height; two coats on walls and one on the ceiling)
- Paint Calculator — Sherwin-Williams (a gallon typically covers about 350–400 square feet; primer 200–300)
- Paint Calculator — Benjamin Moore (these calculations assume 2 coats of paint and do not include the ceiling)
- Waterborne Ceiling Paint Ultra Flat 508, Technical Data Sheet — Benjamin Moore (spread rate 400–450 sq ft per gallon; apply one or two coats)
- Estimating How Much Paint to Buy — Gene & Katie Hamilton, Dummies.com (subtract 20 square feet for each door and 15 square feet for each average-sized window)
Formula and constants last reviewed October 2026
At 2 gallons this is a small-to-average room, so keep the mirror in proportion: one well-placed arch does more for fresh colour than a slab that crowds it.
A 19-inch-wide arch is the right weight for one stretch of wall in a room this size — tall enough to read as architecture, narrow enough to leave the paint you just rolled visible around it.
Check price on Amazon→Read the mirror reviewThe tallest of the three. On a 9-foot wall it fills the vertical the way a real window would, which is the whole point of a window mirror — and it gives a big flat field of fresh colour something to be about.
Check price on Amazon→Read the mirror reviewThirty-two inches of irregular edge break up a large wall without needing a second piece hung beside it, and the gold frame keeps a cool paint colour from going cold.
Check price on Amazon→Read the mirror review- Mirrors that brighten a freshly painted wall — The full roundup these three picks come out of, with the hanging heights.
- Best floor lamps for dark rooms — Wall colour and light work together: a pale wall has a high light reflectance value and hands a lamp back most of what it throws, while a deep saturated colour swallows it. Pick the paint and the lamp as one decision.
- How to place an area rug — Same room, the other horizontal surface — worth settling before the furniture goes back.
We do not review paint, so there is nothing for us to sell you on the gallon count itself — buy whatever your local store mixes well and read the coverage printed on the can. These are the wall pieces we have actually tested and written about.
As an Amazon Associate we earn from qualifying purchases. Picks come from our own published reviews; we never list a product we have not written about.
What it does
The paint calculator tells you how many gallons to buy to cover the walls of a rectangular room — and, if you tick the box, the ceiling too. It works from the four numbers you already have a tape measure for: the room's length, width, and ceiling height, plus how many doors and windows interrupt the walls.
How to use it
Measure the room and enter the length and width in feet, then the ceiling height (8 feet is standard in most homes built in the last few decades). Set the number of doors and windows — these are openings you won't paint, so the calculator subtracts them. Finally choose your number of coats. Two is the honest default for almost any job; you'll only need one if you're refreshing the same color, and you may want three when a pale shade is going over something dark. If you're painting the ceiling too, tick "Also paint the ceiling" — it is figured separately, at one coat, and reported as its own line.
The big number on the right is what to buy. The smaller line items below it show the work: your total wall area, the square footage removed for doors and windows, and the exact (un-rounded) gallons before we round up.
The formula, explained
Here is the entire calculation, which the tool prints live as you type:
gallons = ceil( (2 × (L + W) × H − 20 × doors − 15 × windows) × coats ÷ 400 )
Read it left to right and it's just common sense:
Wall area is the perimeter of the room times its height. The perimeter of a rectangle is 2 × (length + width), and multiplying by ceiling height "unrolls" all four walls into one flat rectangle of paintable surface.
Openings come off. A standard interior door is about 20 square feet of wall you don't have to paint, and a typical window about 15. Subtracting them keeps the estimate from running high. Guides genuinely differ on the window figure: This Old House deducts 10 square feet, while Behr and Home Improvement For Dummies use 15. We use 15, which removes more wall and is the more conservative of the two.
Coats multiply. Two coats means painting that surface twice, so the area to cover doubles before we divide.
Coverage divides. One US gallon of interior wall paint covers roughly 350 to 400 square feet per coat — we divide by 400, the figure This Old House works with and the top of the range Sherwin-Williams quotes. Being the optimistic end, it is the best case: textured or porous walls, bare drywall, deep saturated colors and gloss sheens all drink more, and some Behr lines are rated as low as 250. That is why the tool also prints a second, conservative figure at 350 square feet per gallon — if any of those apply to your room, buy to that number instead.
Round up. Paint comes in whole gallons, and you always want a little left in the can for touch-ups, so the result rounds up to the next whole gallon.
The ceiling is its own calculation. Tick the box and the tool adds a second sum:
ceiling gallons = ceil( length × width × 1 coat ÷ 400 )
Two things to notice there. It uses one coat, not two — the rule of thumb This Old House gives is two coats on the walls and one on the ceiling. And its gallons are rounded up separately and then added, rather than rounding the combined total. That is not sloppiness: ceiling paint is a different product in a different can, a dedicated flat white, so leftover wall paint cannot cover the ceiling's last half gallon. When the box is ticked the tool shows you both numbers, because "3 gallons" really means two of wall color plus one of ceiling white.
What the number does not include. This is wall and ceiling paint only. Primer covers far less ground — Sherwin-Williams puts it at 200 to 300 square feet per gallon — and trim, baseboards, window casings and the doors themselves are a separate product and a separate calculation, even though their area has just been deducted from your walls.
A worked example: a 12 × 14 foot room with 8-foot ceilings, one door and two windows, two coats. The walls are 2 × (12 + 14) × 8 = 416 square feet. Take off 20 + 15 + 15 = 50 for the openings and you have 366 square feet of real wall. Two coats makes 732 square feet to cover; divide by 400 and you get 1.83 gallons — which rounds up to two gallons. That matches what an experienced painter would buy on sight, which is exactly the kind of sanity check a calculator should pass.
Buy from one batch
Paint is mixed in batches, and two cans of the "same" color can differ slightly. If the calculator says two gallons, buy both at once so they're from the same run — and have the store shake them together. A textured wall, a deep saturated color, or bare drywall will all drink more than the 400-square-foot estimate, so buy the extra gallon when in doubt.
2. The Area Rug Size Calculator
Even gap of exposed floor on every side. Retailers put the usable range at 8–24 in; 18 in is the common rule of thumb. Drop toward 8 in in small rooms.
Room geometry only — check the answer against your furniture before you buy.
ideal = (roomShort − 2·gap⁄12) × (roomLong − 2·gap⁄12), then snap DOWN to a standard size = (12 − 3) × (15 − 3) = 9.0 × 12.0 ft → 9 × 12 ftStandard sizes: 3×5, 4×6, 5×7, 5×8, 6×9, 7×10, 8×10, 9×12, 10×14, 12×15 ft. We snap DOWN to the largest that still leaves your chosen border on every side, and flag the next size up when it also stays inside the 8–24 in band — if the two are close, size up. In a living room, also make sure the rug reaches under the front legs of every seat.
This calculator sizes a rug to the ROOM only. It subtracts your chosen border of bare floor twice from each dimension — once per wall — then snaps DOWN to the largest standard rectangular size that still fits inside that footprint. The 18-inch default and the 8-to-24-inch range are retailer guidance, not physics: Rugs.com advises leaving “8 to 24 inches of bare floor between the edge of the rug and the wall”; Home Depot’s rule of thumb is 18 to 24 inches for a furniture-anchored layout and 12 to 18 when the furniture floats; Ruggable suggests 12 to 18. Designers disagree inside that band, so treat the slider as a taste control, not a formula.
What it assumes
- Room geometry only — it takes no furniture input. Home Depot’s own guide warns that “one of the most common reasons that rugs are returned is improper size selection” and that the “appropriate rug size should be determined based on furniture configuration and not on room size alone.”
- It snaps DOWN, so the answer is the largest rug that respects your border, not necessarily the right rug. Both of our rug guides say to size up when you are between two sizes, so the tool also surfaces the next size up whenever that size still leaves at least 8 in of bare floor.
- Only standard US rectangles are on the list: 3×5, 4×6, 5×7, 5×8, 6×9, 7×10, 8×10, 9×12, 10×14 and 12×15 ft. Square, round and runner rugs are not modelled, and 2×3 is excluded because retailers catalogue it as an accent mat, not an area rug.
- In a square or long narrow room a rectangle leaves very uneven borders. Check both border figures: when they differ a lot the tool says so, and a square rug, a round rug, a runner or two rugs is usually the better answer.
- Check the answer against your furniture: the rug should be at least as wide as your sofa with the front legs of every seat on it; a dining rug needs 24 in of clearance beyond the table on every side; a bedroom rug should extend at least 24 in past the bed.
- Tape the dimensions on the floor before you order. Every size here is a nominal size — hand-woven rugs run a couple of inches either way.
Sources
- What Size Rug Do I Need? The Complete Rug Size Guide for Every Room — Rugs.com (standard sizes 3×5 through 12×15; leave 8 to 24 inches of bare floor between rug and wall; front-legs-on rule)
- Selecting Proper Rug Size (PDF) — The Home Depot (18 to 24 inches between wall and rug, equal on all four sides if possible; furniture configuration over room size; bedroom at least 24 inches past the bed)
- Rug Size Guide — Ruggable (sizes include 2×3, 3×5, 4×6, 5×7, 6×9, 8×10, 9×12, 10×14; a 12 to 18 inch border of flooring for an 8×10)
- Rug Size & Placement Guide — Well Woven (leave 18 to 24 inches of exposed floor around the perimeter; stocks 5×7 and 7×10)
- Rug Size Guide — Living Spaces (at least four inches of exposed floor; 2×3 and 2×4 are “small rugs”; dining adds 24 to 30 inches per side)
Formula and constants last reviewed October 2026
Your result is a 9 × 12 ft rug — the big-room end of the lineup. The largest size we have reviewed hands-on is 8 × 10, so these are the closest matches we can stand behind; if your answer is a 9 × 12 or larger, check the seller's size dropdown for the same model before you settle for the 8 × 10.
An 8 × 10 is one tier under your 9 × 12, which means a wider band of bare floor in a 12 × 15 ft room — fine as long as the front legs of every seat still land on it. Low pile and washable is the right trade for the traffic a rug this size takes.
Check price on Amazon→Read the hardwood-floor rug reviewSame 8 × 10 footprint, patterned rather than plain. In a room sized for a 9 × 12 the rug reads as a floor, not an accent, so a medallion hides the traffic lane that a solid field will show.
Check price on Amazon→Read the living-room rug roundupThe natural-fibre look at 8 × 10 without the shedding — worth it when a rug this big sits under a dining table or an open-plan walkway, where a real jute would trap crumbs.
Check price on Amazon→Read the living-room rug roundupA pad should stop about an inch inside each rug edge, so for a 9 × 12 ft rug you cut this one down — Mohawk sells the 8 × 10 as trimmable, and a utility knife does it in a minute. Felt plus rubber is the combination that protects a hard floor.
Check price on Amazon→Read the rug-pad guide- The complete area rug size guide — every standard size room by room, and what to do when you are between two of them
- Do you need a rug pad? — felt, rubber and felt-plus-rubber, and how far inside the rug edge a pad should stop
- How to place an area rug — the front-legs-on rule, dining clearances and bedroom overhang — the furniture check this tool cannot do
Every rug pad we have reviewed is an 8 × 10 — Mohawk and RUGPADUSA both state theirs can be cut to size, and the alternates in our guide (RUGPADUSA Dual Surface, Gorilla Grip felt + rubber) are trimmable too. Cut it an inch smaller than the rug on every side. Sizes shown are the sizes we tested; sellers list other sizes of the same models, so check the size dropdown before you order.
As an Amazon Associate we earn from qualifying purchases. Picks come from our own published reviews; we never list a product we have not written about.
What it does
A rug that's even one size too small is the single most common decorating mistake I'm called in to fix. This calculator takes your room's dimensions and tells you the largest standard rug size that still leaves a clean, even border of bare floor around it — the framing gap that makes a room look composed instead of crowded.
How to use it
Enter the room's length and width in feet. Then set the bare-floor border — the strip of exposed floor you want between the edge of the rug and the walls. The slider defaults to 18 inches, the figure most designers reach for, and runs from 8 inches (right for a small room, where a full 18 would leave a postage stamp of a rug) up to 24 (generous, for large open spaces). That 8-to-24-inch band is Rugs.com's stated rule of thumb; Home Depot's is 18 to 24 inches when furniture sits on the rug and 12 to 18 when it floats. Designers disagree inside the band, so treat the slider as a taste control rather than a formula.
The tool returns the recommended standard size — rugs are sold in fixed dimensions, not made to order, so it snaps down to the nearest real size you can actually buy. It also sorts your two measurements, so the rug's long side always runs along the room's long wall. The line items show your "ideal" footprint before snapping, and the border you'll actually end up with on each side.
Because it snaps down, the headline is the largest rug that keeps your border, not necessarily the rug you should buy. So the tool does two extra things. When the next size up would overshoot your border by only a little and still leave at least 8 inches, it offers that as a size-up option — and between two sizes, sizing up is almost always the better call. And when the two borders come out badly uneven, as they do in a square room or a long narrow one, it says so and names the better answer: a square or round rug, a runner, or two rugs instead of one.
The formula, explained
ideal = (room width − 2 × gap/12) × (room length − 2 × gap/12)
→ then snap down to the nearest standard rug size
The logic is simply "leave a margin on both sides." The gap is in inches, so we divide by 12 to get feet, and subtract it twice from each dimension — once for each wall. What's left is the biggest rug that preserves your border. Because you can't buy a 9.3 × 11.7 rug, the calculator drops to the largest standard size that fits, from the standard US rectangular lineup: 3×5, 4×6, 5×7, 5×8, 6×9, 7×10, 8×10, 9×12, 10×14, and 12×15 feet. It stops there deliberately. A 2×3 is a real size, but retailers catalogue it as an accent mat for a kitchen or an entry rather than a room rug — so instead of recommending one, the tool tells you the room is too tight and points you at a runner or a round rug.
For example, a 15 × 12 foot living room with an 18-inch border: eighteen inches is 1.5 feet, doubled is 3 feet off each dimension, giving an ideal of 12 × 9 feet — which lands exactly on a 9 × 12 rug. A real, buyable size, with a tidy 18-inch frame of floor all the way around.
One rule the math can't see
The border rule sizes a rug to the room. But in a living room you also have to size it to the furniture, and that rule wins when the two disagree. This is not a small caveat — Home Depot's own sizing guide opens by warning that "one of the most common reasons that rugs are returned is improper size selection" and that rug size "should be determined based on furniture configuration and not on room size alone." The calculator can only see geometry, so check its answer against the guideline professionals use:
- The rug should be at least as wide as your sofa, and ideally extend 6 to 12 inches past each arm — guides vary across that range.
- At least the front legs of every seat — sofa and chairs — should sit on the rug. That's what visually ties the seating group into one island. "All legs on" looks even more luxurious if the room is big enough for it.
- In a dining room, the rug needs to extend about 24 inches beyond the table on every side so chairs stay on the rug when they're pulled out.
- In a bedroom, aim for at least 24 inches of rug showing past both sides and the foot of the bed, which is Home Depot's stated minimum. That usually means an 8×10 under a queen and a 9×12 under a king.
If you want to go deeper on material, pile, and styling, our guide to stylish living-room rugs walks through the picks I recommend most often.
Test it with tape
Before you commit, mark the calculator's recommended size on the floor with painter's tape and live with it for a day. Walk the room, pull the dining chairs out, sit on the sofa. It costs nothing and it's the surest way to know a size is right before a rug arrives rolled up at your door.
3. The Room Lighting Calculator
Sets the target brightness in foot-candles.
Applies a rule-of-thumb multiplier, not a standard.
A floor, not a shopping figure — fixtures lose 35–60% of this on the way to the room.
minimum on surface = area × foot-candles
plan to buy = minimum × 1.5 to 2 · bulbs = ⌈ lumens ÷ bulb output ⌉= 192 × 15 = 2,880 lm on the surface
= 4,320 to 5,760 lm to buy · ⌈ 2,880 ÷ 800 ⌉ = 4 bulbs at the bare minimumOne foot-candle is one lumen per square foot, so floor area × target foot-candles gives the light that has to land on the surface — not the lumens to buy. Real fixtures deliver only about 40–65% of their rated lumens into the room, because this shorthand leaves out the fixture’s coefficient of utilisation and the light-loss factor, so treat the headline as a floor and shop for 1.5–2× it. The ceiling factor — 1.0 at 8 ft, 1.2 at 10 ft, 1.5 at 12 ft and above — is a commonly used rule of thumb of roughly 10–15% more lumens per foot above an 8 ft ceiling, not a published standard: the foot-candle charts assume an 8 ft ceiling, a full lumen-method calculation handles height through the room cavity ratio and the fixture’s own photometric data, and a vault well above 12 ft needs more than 1.5×. This sizes the ambient layer only — counters, desks and vanities still want their own task lighting, and dark walls can need twice the output.
Estimate This is an estimate, not a lighting design. The headline is the minimum light that has to LAND on the surface, worked from the consumer shorthand that one foot-candle is one lumen per square foot — it is not the number of bulb lumens to buy. The shorthand leaves out fixture efficiency (the coefficient of utilisation) and light loss, and real fixtures deliver only about 40–65% of their rated lumens into the room. So the figure is a floor: plan on buying roughly 1.5 to 2 times as many lumens, and more in a room with dark walls or heavy shades.
In words: floor area multiplied by a target foot-candle level gives the light that must land on the surface. A real lumen-method calculation then divides that figure by a coefficient of utilisation — how much of a fixture’s output actually reaches the plane, typically 0.5 to 0.8 depending on fixture type, room proportions and wall, ceiling and floor reflectance — and by a light-loss factor for lamp lumen depreciation and dirt, typically 0.7 to 0.85. Together those divisors are roughly 0.35 to 0.68, which is where the 1.5–2× shopping figure comes from.
What it assumes
- The foot-candle targets are midpoints of the residential charts published by Alcon Lighting and reproduced by The Home Depot and 1000Bulbs, with laundry/garage and bathroom-general from Super Bright LEDs. These are consumer summaries of Illuminating Engineering Society guidance, not the IES standard itself: the authoritative document, ANSI/IES/ALA RP-11, is paywalled and varies its targets by occupant age, and the published charts genuinely disagree — bedroom is given as 10–20 foot-candles by some and 20–50 by others, bathroom as 20–50 or 70–80.
- Those charts assume an 8-foot ceiling. The ceiling factors here — 1.0 at 8 ft, 1.2 at 10 ft and 1.5 at 12 ft and above — are a widely used rule of thumb of roughly 10 to 15 percent more lumens per foot above 8 feet, NOT a published standard. No IES, DOE, ENERGY STAR or ALA document publishes a ceiling-height lumen multiplier. A true calculation handles ceiling height through the room cavity ratio and the fixture’s own photometric data, and a vault well above 12 feet needs more than 1.5×.
- The targets are task or work-plane levels, applied here to floor area, so the result sizes the AMBIENT layer only — counters, desks and vanities still need their own task lighting on top.
- The bulb equivalences (450 lm = 40 W, 800 = 60 W, 1,100 = 75 W, 1,600 = 100 W) are the ENERGY STAR minimum-lumen figures for replacing an incandescent bulb, not typical output.
- Light is assumed to be spread across several fixtures. One bright source in the middle of a room will not feel like the same number of lumens.
- Safety note: a hallway or stair is the place to round up rather than down. The 7 foot-candle hallway target sits inside Archtoolbox’s IESNA relay of 5–10 for corridors and stairways, but because this shorthand under-states delivered lumens, a passage built to the bare number can measure well under target in use. There is no stairway setting here — light stairs generously, at both the top and the bottom.
Sources
- Lighting Made Easy (consumer factsheet) — ENERGY STAR (40 W = 450 lm, 60 W = 800 lm, 75 W = 1,100 lm, 100 W = 1,600 lm (minimum lumens))
- Shopping for Light Bulbs — US Federal Trade Commission (a traditional 60-watt incandescent bulb produces about 800 lumens)
- Recommended Lighting Levels in Buildings — Archtoolbox (IESNA Lighting Handbook relay) (cited here only for 1 fc = 1 lumen per square foot and Corridor — General 5–10 fc; it has no residential rows)
- How Many LED Lumens Will I Need? (residential foot-candle chart) — Alcon Lighting (Living Room 10–20, Kitchen General 30–40, Kitchen Stove/Sink 70–80, Dining 30–40, Bedroom 10–20, Hallway 5–10, Bathroom 70–80, Offices Reading and Writing 50–75)
- Determining Lumens (PDF guide) — Alcon Lighting (we based our calculations on 8-foot ceilings; dark walls or shades need roughly 10 more lumens per square foot)
- Lumens Per Square Foot: Recommended Amount Needed for a Room — The Home Depot (living room and bedroom 10–20 fc, kitchen 30–40, work areas 70–80, dining 30–40, bathroom 70–80; dark walls or high ceilings may need extra lumens)
- Residential Recommended Lighting Levels — Super Bright LEDs (Bathroom 20–50; Basement Laundry 20–100; Garage Workshop 20–75; Bedroom 20–50; Dining 10–20)
- Lumen method (zonal cavity) — Wikipedia (required lamp lumens = (A × E) / (CU × LLF))
- Lighting for Interior and Exterior Residential Environments (ANSI/IES/ALA RP-11-26) — Illuminating Engineering Society (the actual IES residential standard; paywalled, with recommendations keyed to occupant visual age)
Formula and constants last reviewed October 2026
Your room needs at least 2,880 lm on the surface, so plan on buying about 4,320–5,760 lm of bulb output. That is a lot of light for a plug-in to carry, which puts you in torchiere territory — uplights bounce off the ceiling and spread. Lumen figures are each maker's own, taken from our published reviews.
A maker-stated 4,000 lm makes this the brightest single source in our reviews, and a 4,320 lm target needs that scale. As a torchiere it bounces off the ceiling, which spreads the light instead of making one hot spot. Two of them reach 4,320 lm, so budget for the pair rather than one.
Check price on Amazon→Read: best lamps for dark roomsThe listing title says 4,200 lm; the maker's own copy gives 3,600 lm for the main uplight plus a separate 6 W reading head, so we count 3,600. The second head covers a chair without lighting the whole room again. Two of them reach 4,320 lm, so budget for the pair rather than one.
Check price on Amazon→Read: best lamps for dark rooms2,500 lm over two switched heads — the smallest of these three, and the one to buy in pairs when you would rather light a big room from two or three corners than from one very bright post. Two of them reach 4,320 lm, so budget for the pair rather than one.
Check price on Amazon→Read: best lamps for dark rooms- Best floor lamps for dark rooms — the brightest plug-in lamps in our reviews, and why one of them is rarely enough
- Lumens, Kelvin and what the numbers mean — why a 2,500 lm lamp can feel dimmer than an 800 lm one
- How to choose a floor lamp — shade, height and placement — the things that decide how much of the output you actually see
One lamp almost never covers a whole room: the figure above is every source in the room added together, which is why each pick says how many it would take. Two or three medium lamps read as better light than one very bright one, and the lumen numbers quoted are the manufacturers’ own ratings — not independent photometric tests — so leave headroom and buy dimmable where you can.
As an Amazon Associate we earn from qualifying purchases. Picks come from our own published reviews; we never list a product we have not written about.
What it does
Most rooms that feel "off" are simply under-lit, and most people have no idea how much light a space needs because light isn't sold the way it's used. This calculator converts a room's size and purpose into a minimum number of lumens — the real measure of brightness — and then tells you roughly how many bulbs it takes to get there. Read "minimum" literally: it is a floor, not a shopping list, and the tool shows you a realistic plan-to-buy figure beside it. More on why in a moment.
How to use it
Enter the room's length and width. Choose the room or task from the menu — a living room and a kitchen counter have very different brightness needs, and the tool knows the target for each. Set your ceiling height, since taller rooms lose light on the way down, and pick the bulb brightness you plan to use. The result is the total lumens the space wants, plus the number of fixtures of that brightness it takes to deliver them.
The formula, explained
The calculation is two short steps:
lumens = area × foot-candles × ceiling factor
bulbs = ceil( lumens ÷ bulb output )
This rests on one tidy definition: one foot-candle is one lumen falling on one square foot. So if you know how brightly lit a surface should be — its target in foot-candles — you just multiply by the floor area to get the total lumens the room needs. The targets baked into the menu are the midpoints of the residential foot-candle charts published by Alcon Lighting and reproduced by Home Depot and 1000Bulbs, with the laundry/garage and general-bathroom rows from Super Bright LEDs:
- Living room and bedroom: about 15 foot-candles — relaxed, ambient.
- Kitchen and dining: about 35 — alert, working light.
- General bathroom: about 35, which is the one figure here worth a caveat — Super Bright LEDs' 20-to-50 band supports it, but Alcon and Home Depot list bathrooms at 70 to 80 with no general-versus-vanity split. If your bathroom has no separate vanity light, aim higher.
- Kitchen counters and bathroom vanities: 75 — detailed task work, where you really need to see.
- Home office / desk: about 60.
- Laundry and garage: about 40.
- Hallways: around 7, just enough to move through safely.
A word on provenance, because it matters. These charts summarise Illuminating Engineering Society guidance, but they are consumer relays, not the standard itself. The IES's own residential document (ANSI/IES/ALA RP-11) is paywalled and varies its targets by the occupant's age, so none of these numbers can be checked against it directly — and the published charts genuinely disagree with each other. Bedrooms are given as 10 to 20 foot-candles by some and 20 to 50 by others. Treat the menu as a sensible consensus, not gospel.
The ceiling factor multiplies the result by 1.0 at 8 feet, 1.2 at 10 feet and 1.5 at 12 feet and above. Be aware that this one is a rule of thumb — roughly 10 to 15 percent more light per foot above an 8-foot ceiling — and not a published standard. The foot-candle charts all assume 8-foot ceilings (Alcon says so explicitly), and a real lighting calculation handles height through the room's cavity ratio and the fixture's own photometric data. A vault well above 12 feet needs more than 1.5 times.
Then it's a matter of dividing by how much light each bulb gives off. Because bulbs are still sold in old wattage habits, here's the translation the calculator uses: a 60-watt-equivalent LED is about 800 lumens, a 75-watt-equivalent about 1,100, and a 100-watt-equivalent about 1,600. Divide the room's total by the bulb's output, round up, and you have your fixture count.
A worked example: a 12 × 16 foot living room is 192 square feet. At 15 foot-candles that's 192 × 15 = 2,880 lumens. With 800-lumen (60-watt-equivalent) bulbs, 2,880 ÷ 800 = 3.6, which rounds up to four bulbs. A single ceiling fixture was never going to do it — and now you know exactly why a four-source plan feels so much better.
Why the tool also shows a bigger number
Here is the honest limitation, and it is the reason the headline says minimum. Multiplying area by foot-candles tells you the light that has to land on the surface. It does not tell you how many bulb lumens to buy, because no fixture delivers all of its rated output to the floor. A real lumen-method calculation divides by two more terms: a coefficient of utilisation (how much of a fixture's light actually reaches the plane, typically 0.5 to 0.8 depending on the fixture, the room's shape and how reflective the walls and ceiling are) and a light-loss factor (lamp output fading with age, plus dirt, typically 0.7 to 0.85).
Multiply those out and real fixtures put only about 40 to 65 percent of their rated lumens where you want them. That is why the tool prints a plan-to-buy figure of roughly 1.5 to 2 times the bare minimum, and why you should shop to that number. Our 2,880-lumen living room really wants something closer to 4,300 to 5,800 lumens of bulbs.
Read this before you trust the number
The lighting figure is the roughest estimate of the three, so a few caveats:
- It is a minimum, not a target. As above, the shorthand ignores fixture efficiency and light loss, so buy to the plan-to-buy figure rather than the headline.
- It sizes the ambient layer only — the general fill of light. Work zones like counters, desks, and mirrors still want their own dedicated task lighting on top, which is why those targets are so much higher.
- Dark walls and ceilings absorb light. A room with deep, moody surfaces can need up to twice the lumens of a pale one to feel equally bright.
- Spread the light out. Four modest sources around a room always beat one blazing fixture in the middle — softer shadows, no glare, more flattering. Layer ambient, task, and a little accent light, and a room comes alive. A pair of well-placed floor lamps is one of the easiest ways to add a layer without rewiring anything.
Lumens, not watts
When you shop, ignore the wattage and read the lumens on the package — watts measure energy used, not light produced, and a modern LED makes far more light per watt than the old bulbs the equivalents are named after. The number the calculator gives you is in lumens precisely so it survives whatever bulb technology you buy.
How accurate are these, really?
These calculators are built on the same formulas and industry figures the major paint, rug, and lighting brands use — paint coverage quoted at 350 to 400 square feet per gallon (we divide by 400, the top of that range, and show a conservative 350 figure alongside it), the 18-inch rug border, one foot-candle per square foot — and each one has been checked against worked examples from those sources so the answers land where an experienced eye would put them. Every constant they use is listed, with a link to the source that states it, in the "Where these numbers come from" panel inside each tool.
Two limits are worth naming plainly rather than burying. The rug tool sees room geometry only and snaps down, so its answer is a floor on the size and your furniture layout overrules it. The lighting tool uses the consumer shorthand and omits fixture efficiency and light loss, so its headline is a minimum and the plan-to-buy figure is the one to shop with. The ceiling-height multiplier it applies is a rule of thumb, not a standard.
But they are estimates, and they're meant to be. A room is never a perfect rectangle, walls have quirks, and your taste is a variable no formula contains. Treat the result as a confident starting point: the number that tells you whether to buy two gallons or four, whether you're shopping for an 8×10 or a 9×12, whether one fixture will do or you need four. That's exactly the decision most people get wrong by guessing, and exactly the one a little arithmetic gets right.
Frequently asked questions
Do I need to create an account or download anything? No. All three calculators run in your browser, free, with no sign-up and nothing to install. Type your numbers and the answers appear instantly.
Why does the paint calculator round up to a whole gallon? Because you always want a little left over for touch-ups after the room is lived in, and because one trip to the store beats two. It does mean a job that lands just over a whole number — 2.01 gallons, say — rounds to 3. If the tool shows a large fraction, ask the store about buying the remainder as a quart instead of a full gallon.
My room isn't a rectangle. Can I still use these? Yes — split an L-shaped or irregular room into rectangles, run each through the calculator, and add the results. For paint, total the gallons; for lighting, total the lumens — but if the room has one tall ceiling throughout, apply the ceiling-height setting once rather than to each sub-rectangle, or you will double-count it. The rug tool is best used on the main rectangular zone you're trying to anchor.
What if the rug calculator says my room is "too tight"? That means no standard rectangular rug leaves your chosen border in a room that small. Lower the border toward 8 inches, or consider a runner or a round rug — both work beautifully in compact spaces where a big rectangle would overwhelm. The calculator only handles rectangles, so it will tell you to look at those shapes rather than sizing one for you.
Should I measure in feet or inches? Enter room dimensions in feet (decimals are fine — a 12-foot-6-inch wall is 12.5). The rug border is the one input in inches, because that's how designers talk about that small a gap.
Measure once, run the numbers, and buy with confidence. Bookmark this page — between a coat of paint, a new rug, and better lighting, it covers the three projects that change a room the most for the least.
The Author
About the author
Interior Designer
Donald Martin
Designer since 2006 and a lifelong lover of rescued objects, Donald Martin knows how to give a second life to pieces and weave them into an eclectic, personalized interior. He writes about decorating real homes on a real budget.
