Paint makers say a gallon covers 350 to 400 square feet. The hard part is working out your square feet and how many coats. This does both, for walls, ceiling and trim at once, and tells you which cans to buy.
Paint calculator
Enter the room. We round up to the cans you can actually buy.
The math
- Wall area = 2 x (length + width) x ceiling height.
- Subtract about 21 sq ft for each door (3 x 7 ft) and 15 sq ft for each window (3 x 5 ft).
- Gallons = area x coats ÷ coverage. We use 350 sq ft per gallon per coat unless you change it.
- Round up to the quart: three quarts cost about as much as a gallon, so we switch to a gallon instead.
Coverage, coats and primer
- Sherwin-Williams says a gallon covers about 350 to 400 square feet and a quart about 100. Rough, porous or textured walls take more.
- Two coats is normal. One coat only works for the same color over a clean, sound wall.
- Primer covers less, about 200 to 300 sq ft per gallon. Use it on new drywall, patched areas, stains and big color changes.
Example
A 14 x 12 ft room with 8 ft ceilings, one door and two windows has 2 x 26 x 8 = 416 sq ft of wall, minus 21 and 30 = 365 sq ft. Two coats at 350 sq ft per gallon is 2.09 gallons: buy 2 gallons and 1 quart.
Sources
Sources last checked 1 November 2026.
Frequently asked questions
How many square feet does a gallon of paint cover?
Paint makers say 350 to 400 square feet per coat on smooth, primed walls. Use the lower number for textured or porous surfaces.
Do I need two coats of paint?
Usually yes, for even color. One coat can do when repainting the same color.
How much paint for a ceiling?
Length times width, divided by coverage, times coats. A 12 x 12 ceiling with two coats needs about 1 gallon.