Paint Calculator
A 12 ft by 10 ft room with 8 ft walls, two coats, 21 sq ft of doors and windows, 350 sq ft of coverage per gallon, and 10% waste needs about 2.08 gallons of paint, or 7.88 liters. This paint calculator turns a room measurement into a practical paint estimate for wall surfaces. It uses the full room perimeter, subtracts openings you do not plan to paint, multiplies by the number of coats, then applies your paint coverage and waste setting. The result is meant to answer the everyday question people actually have before a project starts: how many gallons should I plan to buy for this room?
Quick answer
The calculator treats the room as one rectangle, so wall area starts with perimeter times wall height.
What this tells you
- •The calculator treats the room as one rectangle, so wall area starts with perimeter times wall height.
- •It subtracts one combined non-paint area entry for doors, windows, and any other openings you want excluded.
- •It multiplies the remaining wall area by the number of coats because two coats cover twice the surface area of one coat.
- •It divides that coated area by the coverage rate you enter, usually the square footage per gallon from the paint can or product sheet.
- •It adds your waste allowance at the end to cover roller loss, touch-ups, porous walls, and the small amount that never makes it back into the can.
- •The result is a decimal estimate in gallons and liters. In real buying, most people round up to the next full can size instead of trying to buy the exact decimal amount.
How to Use
- 1Measure the room length and room width in feet. Use the inside wall dimensions of the room you are painting, not the floor area of the whole house.
- 2Enter the wall height in feet. For a room with a sloped ceiling or knee wall, use the average paintable wall height only if that average is a fair simplification.
- 3Choose the number of coats. Two coats are common for color changes, new drywall, and many finish paints. One coat can be enough for a quick refresh when the old color is close and the paint covers well.
- 4Check the paint label for coverage per gallon and enter that number instead of guessing. Smooth walls with quality paint often cover more than rough walls, deep colors, or primer-heavy jobs.
- 5Add up the area of doors, windows, large mirrors, built-ins, or other surfaces you will not paint, then enter that total as the non-paint area.
- 6Set a waste percentage. A simple repaint on smooth walls may need less extra paint than a first coat on patchy drywall, a dark-to-light color change, or a room with lots of cut-in work.
- 7Click calculate and read the net wall area plus the estimated gallons and liters. If you are buying cans, round up to a practical purchase size after comparing the result with the can sizes available at your store.
How It Works
Formula
Perimeter = 2 x (length + width)
Gross wall area = perimeter x wall height
Net wall area = gross wall area - non-paint area
Paint needed = (net wall area x coats / coverage per gallon) x (1 + waste%)The math follows the way most people measure a basic rectangular room. First, the calculator finds the wall perimeter from the room length and width. It multiplies that perimeter by wall height to get total wall square footage. Next, it subtracts the total area of doors, windows, and other openings you entered. That gives the net wall area that actually receives paint. Because every additional coat covers the same wall surface again, the calculator multiplies the net area by the number of coats. It then divides by the paint coverage rate to estimate raw gallons and adds your waste percentage so the final number reflects normal job loss instead of a perfect lab result.
Calculation note: values are processed in the order shown above, using the current input units.
Worked Examples
Small bedroom repaint
The perimeter is 44 feet, so gross wall area is 44 x 8 = 352 square feet. After subtracting 21 square feet for a door and window area total, the paintable wall area is 331 square feet. Two coats means 662 square feet of coated area. Divide by 350 and add 10% waste, and the estimate lands at about 2.08 gallons. In practice, this is the kind of room where many people buy 3 gallons if they want a cushion for touch-ups.
Living room with taller walls
The room perimeter is 60 feet. At 9 feet high, gross wall area is 540 square feet. Subtract 40 square feet of windows and doors and you have 500 square feet to paint. Two coats produce 1,000 square feet of coated area. At 325 square feet per gallon, raw paint need is about 3.08 gallons. After a 12% allowance for waste and touch-up margin, the estimate becomes about 3.45 gallons.
Home office color refresh
An 11 by 11 room has a 44-foot perimeter. Multiply by an 8-foot wall height for 352 square feet of gross wall area. Subtract 18 square feet of openings and 334 square feet remain. Because this is a one-coat refresh, the coated area stays 334 square feet. Dividing by 375 square feet per gallon gives about 0.89 gallons. With 8% extra, the estimate reaches about 0.96 gallons, which tells you one gallon is close but may be tight once cut-in and roller loss are factored in.
New nursery over patched drywall
The perimeter is 52 feet, so gross wall area is 416 square feet. After subtracting 24 square feet of openings, the net wall area is 392 square feet. Two coats means 784 square feet of coated area. At 300 square feet per gallon, raw paint use is about 2.61 gallons. A 15% allowance is reasonable here because fresh patches and new wall repairs tend to absorb more paint than a simple maintenance repaint.
Guest bedroom repaint in two coats
Marcus is taking a guest bedroom from a dated beige to a warm white and plans two coats so the old color does not show through. The room measures 13 by 11 feet with 8 foot walls, which gives a 48 foot perimeter and 384 square feet of gross wall area. He subtracts 55 square feet for the entry door, the closet door, and one window, leaving 329 square feet to paint. Two coats make 658 square feet of coated area. His paint's technical data sheet lists 400 square feet per gallon, so the raw need is 1.645 gallons, and the 10% waste allowance lifts it to 1.81 gallons, or 6.85 liters. Two one-gallon cans finish the room with some left for touch-ups. A single gallon would run out partway through the second coat.
Gallons for common rooms at this calculator's coverage rate
Wall paint for eight common room sizes, computed with the formula above at the calculator's default settings: 350 sq ft per gallon, no doors or windows subtracted, and a 10% waste allowance. Each cell is the decimal gallon figure the tool returns. Subtract your openings and the number drops a little, then round up to whole cans before you buy.
| Room (ft) | 8 ft walls, 1 coat | 8 ft walls, 2 coats | 9 ft walls, 1 coat | 9 ft walls, 2 coats |
|---|---|---|---|---|
| 10 x 10 | 1.01 gal | 2.01 gal | 1.13 gal | 2.26 gal |
| 10 x 12 | 1.11 gal | 2.21 gal | 1.24 gal | 2.49 gal |
| 12 x 12 | 1.21 gal | 2.41 gal | 1.36 gal | 2.72 gal |
| 12 x 14 | 1.31 gal | 2.61 gal | 1.47 gal | 2.94 gal |
| 12 x 15 | 1.36 gal | 2.72 gal | 1.53 gal | 3.05 gal |
| 14 x 16 | 1.51 gal | 3.02 gal | 1.70 gal | 3.39 gal |
| 15 x 20 | 1.76 gal | 3.52 gal | 1.98 gal | 3.96 gal |
| 16 x 20 | 1.81 gal | 3.62 gal | 2.04 gal | 4.07 gal |
Computed with the formula above: 2 x (length + width) x wall height x coats, divided by 350, then multiplied by 1.10 for waste and rounded to two decimals. Coverage per gallon comes from each product's technical data sheet and varies with surface texture, porosity, and how far the color is changing. The calculator uses whatever figure you enter, and 350 sq ft per gallon is only its starting default.
What this paint estimate assumes
This calculator is built for wall paint in a simple rectangular room. It does not calculate ceiling paint, trim paint, cabinets, doors, baseboards, or exterior siding. If you are painting those surfaces too, treat them as separate jobs with their own measurements and product coverage rates.
The biggest judgment call is the coverage number. Paint cans often list a spread range because smooth sealed walls behave differently from bare drywall, old flat paint, patched areas, brick, or textured surfaces. If your walls are rough or thirsty, using the optimistic end of the range will make the estimate look better than the project actually goes. A cautious coverage input usually leads to fewer surprises.
Waste also matters more than people expect. A calculator can tell you the theoretical paint volume, but roller trays, cut-in around trim, touch-up work, and the paint left in brushes and liners all add up. That is why the result is most useful as a planning number. When you go to the store, compare the result with real can sizes and buy enough to finish the room without hoping the last wall stretches farther than the math says.
Two exact definitions sit under the arithmetic. A US gallon is 231 cubic inches (NIST Handbook 44, Appendix C), which is why a coating that was 100 percent solids would cover about 1,604 square feet at a dry film one thousandth of an inch thick. Real wall paint carries far less solid material, so the spread rate on a technical data sheet already accounts for the water or solvent that evaporates. The liters figure uses 3.785 liters per gallon (NIST Special Publication 811) and is rounded to two decimals.
Four assumptions set the limits of the estimate. The coverage figure is the one you enter, and manufacturers publish it as a range because texture, porosity, and color change all alter how far a gallon goes, so a textured or patched wall will use more than the math shows. Primer and ceiling paint are separate products with their own spread rates and are not included. Cutting in around trim, outlets, and corners wastes paint that the waste percentage only approximates. And paint is sold in whole cans, so a result of 1.81 gallons means buying 2 gallons, not 1.81.
Common mistakes
- Measuring floor area and calling it wall area. Paint goes on the perimeter walls, so a 12 by 10 room is not the same as 120 square feet of paintable wall.
- Leaving the default coverage rate in place when the product label gives a different number. Premium paints, primers, textured surfaces, and deep-color bases do not all spread the same way.
- Forgetting that two coats double the coated area. A room that barely fits in one gallon on paper can jump above two gallons once you add the second coat and waste.
- Subtracting too much for openings. Unless you are skipping trim and edges entirely, small windows and doors do not remove as much usable paint area as people sometimes think.
- Treating the decimal result as a store-ready purchase list. The calculator returns paint volume, not the exact mix of gallon, quart, and sample sizes your supplier stocks.
- Using the same waste percentage for every room. Smooth white-over-white repaints are different from rough walls, dark colors, and first coats over new compound.
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