Paint calculator

Gallons of paint for a room's walls, and the ceiling if you want, from its size, openings and coats.

Formula and sources last checked October 2, 2026. Found a mistake? Report an error.

How it's calculated

The calculator measures the wall area of a rectangular room, subtracts doors and windows, adds the ceiling if you tick it, and divides by the coverage on your paint label. Coverage varies by product and surface, so use the number on your can or data sheet if you have it.

wall area = 2 × (length + width) × height
area to paint = wall area − doors × door size − windows × window size (+ length × width for the ceiling)
gallons = area to paint × coats ÷ coverage

Worked example

A 12 ft × 10 ft room with 8 ft walls has 2 × 22 × 8 = 352 sq ft of wall. Minus one door (20 sq ft) and two windows (30 sq ft) leaves 302 sq ft. Two coats at 350 sq ft per gallon need 302 × 2 ÷ 350 = 1.73 gallons, so buy 2 gallons.

The default coverage of 350 sq ft per gallon is the low end of a typical manufacturer range. Rough, porous or dark-to-light color changes may need more paint or a primer.

Sources

1. Coverage 350–400 sq ft per gallon at recommended film thickness (example product): Benjamin Moore ULTRA SPEC 500 technical data sheet. Check the data sheet of the paint you buy.
2. Door and window sizes are typical defaults, not from a standard; measure yours for a closer estimate.