This room paint calculator turns a rectangular room into a gallon count. You enter the length and width of the floor, the wall height, the square feet of doors and windows, the number of coats, and how many square feet one gallon covers. You also say whether the ceiling is painted. The sample is a 12 by 10 foot room with 8 foot walls, 36 square feet of openings, two coats, 350 square feet per gallon, and the ceiling included. The headline is the number of whole gallons to buy.
The arithmetic is area, not a price. It does not pick a brand, a sheen, or a color, and it does not know whether the walls were already painted a similar color. Primer, texture, bare drywall, and a drastic color change can all use more paint than the coverage number on a can of finish paint. The concrete slab calculator on this site answers a different shopping question, in cubic yards and bags. Do not use a bag yield as a gallon coverage, or the reverse.
How to use this calculator
- Enter length, width, and wall height in feet. The sample is 12 by 10 with walls 8 feet high. Measure the room you are painting, including a closet only if you will paint it.
- Enter the combined square feet of doors and windows. A common 3 by 7 foot door is 21 square feet. The sample, 36, is that door plus about 15 square feet of window. Use 0 if you will paint the openings too.
- Enter the number of coats and the coverage in square feet per gallon. The sample is 2 coats and 350. Then choose whether the ceiling is included.
- Press Calculate. The headline rounds up to a whole gallon. The line under it keeps the exact gallons, and another line rounds up only as far as the next quart.
How room area becomes a gallon count
Wall area is the perimeter times the height. The perimeter of a rectangle is twice the length plus the width, so a 12 by 10 foot room has a perimeter of 44 feet. At 8 feet high the walls are 44 × 8 = 352 square feet. The ceiling, if you include it, is length times width, 120 square feet. Openings are subtracted from the walls only. They are not subtracted from the ceiling, because a door is not a hole in the ceiling. Paintable area in the sample is 352 − 36 + 120 = 436 square feet.
Each coat covers that area again. Two coats mean 872 square feet of paint. Divide by the coverage you typed. At 350 square feet per gallon, 872 / 350 = 2.491… gallons, which the screen shows as 2.49. You cannot buy 0.49 of a gallon from the whole-gallon line. The headline rounds up to 3 gallons unless the division is exact. The quart line is less blunt: 2.49 gallons rounds up to 2.5 gallons, which is 10 quarts. If the store sells quarts, 2 gallons plus 2 quarts is closer to the math than 3 gallons. The headline still says 3, because a gallon is the unit it promises to round to.
Coverage is your input, not a hidden constant. A can that says 400 square feet per gallon will not match a can that says 350. The sample uses 350 because that is a common figure for smooth finished walls. Porous drywall, bare wood, and heavy texture drink more, so a lower coverage number is the honest input, not a second waste percentage. This form has no separate waste field. If you want slack, lower the coverage or add a fraction of a coat. Turning 2 coats into 2.25 is allowed. The coat field is not limited to whole numbers.
Leaving the ceiling off changes the sample more than people expect. The walls after openings are 316 square feet. Two coats are 632 square feet, divided by 350 is 1.81 gallons. The headline becomes 2 gallons, and the quart line is exactly 2 gallons, which is 8 quarts. The ceiling was 120 square feet, 240 square feet across two coats, about 0.69 gallon at this coverage. That is why the with-ceiling exact figure is 2.49 rather than 1.81. Painting the ceiling is a real extra purchase, not a rounding artifact.
Formula
Perimeter = 2 × (length + width) Wall area = perimeter × height Ceiling area = length × width, or 0 if the ceiling is not painted Paintable area = wall area − openings + ceiling area Openings cannot exceed the wall area Exact gallons = paintable area × coats / coverage Gallons to buy = exact gallons, rounded up to the next whole gallon Quarts to buy = exact gallons × 4, rounded up to the next whole quart
The sample is exact: walls 352, ceiling 120, openings 36, paintable area 436, two coats 872, 872 / 350 = 2.491428…, displayed as 2.49 gallons. The whole-gallon headline is 3. The quart line is 2.5 gallons, or 10 quarts. With the ceiling turned off, 316 × 2 / 350 = 1.8057…, displayed as 1.81 gallons, and the headline is 2.
An 8 by 8 foot room with 8 foot walls, no openings, one coat, walls only, and 350 square feet per gallon is 256 / 350 = 0.731… gallons, displayed as 0.73. The headline is still 1 gallon, because that is the whole-gallon round-up. The quart line is 0.75 gallons, or 3 quarts. On a room this small the quart line is the more useful shopping number, and the headline is there so a gallon-only store is not short.
Worked room paint examples
12 by 10 feet, 8 foot walls, two coats, ceiling included
The calculator shows 3 gallons to buy. Exact paint is 2.49 gallons. Rounded up to the next quart, that is 2.5 gallons, or 10 quarts. Wall area is 352 square feet, the ceiling adds 120, and 36 square feet of openings come off the walls. Paintable area is 436 square feet across 2 coats. Coverage is 350 square feet per gallon.
Buying 3 gallons covers the round-up and leaves almost half a gallon unopened if the walls take paint the way the can promises. Buying 2 gallons and 2 quarts matches the quart line and leaves less leftover. Neither answer prices the paint. A third coat, or a primer that the can does not count inside the 350, is a new calculation. Change the coat field rather than guessing a percentage on top of the 3.
The same room without the ceiling, and one coat at 400 square feet per gallon
Turn the ceiling off and keep two coats at 350. The headline is 2 gallons to buy. Exact paint is 1.81 gallons, and the quart line is 2 gallons, or 8 quarts. Wall area stays 352 square feet. Openings stay 36. Paintable area is 316 square feet. The ceiling’s 120 square feet were the difference between 2.49 gallons and 1.81 gallons before the round-up. After the round-up the shopping difference is one gallon, from 3 down to 2.
Walls only, no openings, one coat, and a can that covers 400 square feet per gallon: exact paint is 0.88 gallons, the headline is 1 gallon, and the quart line is 1 gallon, or 4 quarts. The walls are still 352 square feet. One coat at 400 uses less than one coat at 350 would, and far less than two coats. If the can in your hand says 350, do not leave 400 in the form because the sample started there. The coverage field is the claim printed on that can.
A 10 by 10 room with one door, and a larger room that needs five gallons
A 10 by 10 foot room, 8 foot walls, 21 square feet of openings, two coats, 350 square feet per gallon, and the ceiling included, shows 3 gallons to buy. Exact paint is 2.28 gallons, or 2.5 gallons at the quart line, which is 10 quarts. Wall area is 320 square feet and the ceiling is 100. Paintable area is 399 square feet. One standard door, and no window subtracted, still does not bring a two-coat ceiling job under 2 gallons. The exact figure would have to be 2.00 or less before the headline dropped to 2.
A 15 by 12 foot room, 9 foot walls, 60 square feet of openings, two coats, 300 square feet per gallon, and the ceiling included, shows 5 gallons to buy. Exact paint is 4.04 gallons. The quart line is 4.25 gallons, or 17 quarts. Wall area is 486 square feet, the ceiling is 180, and paintable area is 606 square feet across two coats. The lower coverage is doing as much work as the larger room. At 350 square feet per gallon this same room would use less. Type the number from the can you will open, especially for a dark color that the label says covers fewer square feet.
Gallons for rectangular rooms at the coverage you set
| Room | Openings | Coats | Coverage | Ceiling | Exact | Gallons to buy | To the next quart |
|---|---|---|---|---|---|---|---|
| 12 × 10 × 8 ft | 36 sq ft | 2 | 350 | Yes | 2.49 | 3 | 2.5 gal (10 qt) |
| 12 × 10 × 8 ft | 36 sq ft | 2 | 350 | No | 1.81 | 2 | 2 gal (8 qt) |
| 12 × 10 × 8 ft | None | 1 | 400 | No | 0.88 | 1 | 1 gal (4 qt) |
| 10 × 10 × 8 ft | 21 sq ft | 2 | 350 | Yes | 2.28 | 3 | 2.5 gal (10 qt) |
| 15 × 12 × 9 ft | 60 sq ft | 2 | 300 | Yes | 4.04 | 5 | 4.25 gal (17 qt) |
| 8 × 8 × 8 ft | None | 1 | 350 | No | 0.73 | 1 | 0.75 gal (3 qt) |
What a rectangular gallon count leaves out
The room is a box. A sloped ceiling, a stair hall, an alcove, wainscot that stops partway up the wall, and built-in shelves that you paint on every face are different areas. Measure those separately and add the square feet, or raise the height only if that honestly represents the extra wall. The form will not accept a triangle. Openings larger than the wall area are rejected, because a subtraction bigger than the walls would invent negative paint.
Coverage on a label is a laboratory-style figure for smooth, sealed surfaces. The first coat on new drywall often covers less. A gallon of primer is not counted unless you include it as a coat or lower the coverage. Dark colors, ceiling paint, and masonry paint publish their own numbers. Trim paint in a quart is outside this wall-and-ceiling area. Closet interiors are excluded unless their length and width are part of the room you measured.
The result is in US gallons and square feet. A UK or Australian litre can is not a US gallon, and a metre-measured room has to be converted to feet before it is typed here. One US gallon is about 3.785 litres. The length converter on this site turns metres into feet if that is the measurement you have. It does not convert the gallon answer into litres. Multiply the exact-gallon line by 3.785 yourself if the can on the shelf is metric, then round up to a can you can buy.
Waste is not a separate percentage. Rounding up to a gallon or a quart is the only spare the formula adds, and it adds that spare so you do not stop short of the area, not so you can repaint after a spill. Two people with the same room and different coverage numbers should get different answers. If they do not, one of them left the sample coverage of 350 in the form.
Frequently asked questions
How many gallons of paint do I need for a 12 by 10 room with 8 foot ceilings?
With 36 square feet of doors and windows, two coats, 350 square feet per gallon, and the ceiling included, this calculator shows 3 gallons. The exact amount is 2.49 gallons, or 2.5 gallons if you can buy quarts. Walls are 352 square feet and the ceiling is 120.
Does the room paint calculator include the ceiling?
Only if you choose “Paint the ceiling.” The sample includes it. Turn it off and the same 12 by 10 room falls from 2.49 exact gallons to 1.81, and the whole-gallon headline falls from 3 to 2. Openings are subtracted from the walls either way. They are not subtracted from the ceiling.
Why does the gallon count round up when the quart line is lower?
The headline is a whole gallon, because that is a common can size. The sample’s 2.49 gallons becomes 3 on that line and 2.5 gallons, 10 quarts, on the quart line. An 8 by 8 foot room at one coat can show 1 gallon in the headline and 0.75 gallons, 3 quarts, underneath. Use the quart line when the store sells quarts.
What coverage per gallon should I type?
Type the number on the can you will use. The sample uses 350 square feet per gallon, which is a common figure for smooth walls and not a rule. A can rated at 400 square feet per gallon, on the sample walls with one coat and no ceiling or openings, uses 0.88 gallons. The same walls at 350 would use more. Porous or textured surfaces need the lower number the job actually gets, not the best number on the label.
How do I count a door and a window?
Add their areas and type one number. A 3 by 7 foot door is 21 square feet. The sample’s 36 square feet is that door plus 15 square feet of window. A 10 by 10 room with only the 21 square foot door, two coats, the ceiling, and 350 square feet per gallon still shows 3 gallons, because the exact amount is 2.28. Use 0 if you are painting the doors and windows the same as the walls.
Can I use this gallon count for a concrete slab or a metric room?
No for the slab. The concrete calculator uses cubic feet, cubic yards, and bag yields. Paint coverage is square feet per gallon. For a room measured in metres, convert the length, width, and height to feet first. The exact-gallon line times 3.785 is an approximate litre count. This form does not sell the paint in litres itself.
Related ClearCalc pages
- Concrete slab calculator Bags and cubic yards for a rectangular pour. A different material and a different unit.
- Floor tile calculator A 10 by 12 foot floor with 12 inch tiles, a 1/8 inch joint, and 10% waste is 130 tiles. Paint on this page is gallons, not a tile count.
- Length converter from meters Convert a metric room to feet before you type the length, width, and height here.
- Percentage calculator Useful if you want to raise the coverage assumption by a percent after you have read the can. This paint form has no separate waste field.