How the estimate works
Wall area is the room's perimeter times the wall height: 2 × (length + width) × height. Subtract the doors and windows you are not painting, multiply by the number of coats, and divide by the coverage per gallon on the can. Then add a little extra and round up to what you can buy.
The default door (21 sq ft) is a 3 × 7 ft opening and the default window (15 sq ft) is 3 × 5 ft. Change them if yours are much bigger, such as a sliding patio door or a picture window. Small openings barely matter: a single extra window changes the answer by about 0.04 gallons per coat.
Coverage per gallon
Manufacturers state a spread rate on each product. Benjamin Moore, for example, lists its Regal Select interior paint at 400 to 450 square feet per gallon, which is why the default here is 400. Use the low end of the range printed on your can: real walls have texture and edges, and rollers leave paint behind.
Coverage is per coat. Two coats are standard for a color change, and very light colors over dark ones (or the reverse) can need a primer or a third coat. If you are painting new drywall, prime first; primer usually has its own coverage figure.
Gallons or quarts?
A US gallon is four quarts, or about 3.79 liters. The result shows both a whole-gallon count and a gallons-plus-quarts option rounded up to the next quart, so you can decide which is the better buy at your store. Buying the whole amount at once from the same batch avoids slight color differences between cans; if you do end up with several cans, mix them together in a larger bucket before you start.
The formulas
Wall area = 2 × (L + W) × HPaintable area = wall area − (doors × door area) − (windows × window area)Gallons = paintable area × coats ÷ coverage × (1 + extra ÷ 100)Ceiling gallons = L × W × coats ÷ coverage × (1 + extra ÷ 100)- L, W = room length and width in feet; H = wall height in feet
- coverage = square feet one gallon covers in one coat
- extra = overage percentage; gallons are then rounded up
Example: a 12 × 12 ft bedroom with 8 ft walls
- Wall area: 2 × (12 + 12) × 8 = 384 sq ft.
- Subtract one 21 sq ft door and two 15 sq ft windows: 384 − 51 = 333 sq ft.
- Two coats at 400 sq ft per gallon: 333 × 2 ÷ 400 = 1.67 gallons.
- Add 10% extra: 1.67 × 1.10 = 1.83 gallons, so buy 2 gallons.
- The 144 sq ft ceiling, two coats: 144 × 2 ÷ 400 × 1.10 = 0.79 gallons, so 1 gallon of ceiling paint.
Frequently asked questions
How much paint do I need for a 12x12 room?
With 8 ft walls, one door, two windows, two coats and 400 sq ft per gallon, about 1.83 gallons including 10% extra, so 2 gallons for the walls and 1 more for the ceiling.
How many square feet does a gallon of paint cover?
It depends on the product and surface. Check the can or the data sheet; Benjamin Moore lists 400 to 450 sq ft per gallon for its Regal Select interior paint. Rough, porous or unprimed surfaces get less.
Do I need two coats of paint?
Usually, when changing color or painting over patches. One coat can be enough when repainting the same color over a clean, sound finish. Some paint labels state that one or two coats are recommended; follow the product's directions.
Should I subtract doors and windows?
Yes for large openings, since they can be a meaningful share of the wall. In the example above, one door and two windows remove 51 of 384 sq ft, about 13% of the wall area.
How much paint for the ceiling?
The ceiling area is simply length × width. A 12 × 12 ft ceiling is 144 sq ft; two coats at 400 sq ft per gallon plus 10% extra is 0.79 gallons, so one gallon.
Sources
Last reviewed for 2026. How we calculate.