How to use this calculator
Measure the four walls of a rectangular room, then add up the area of doors and windows you will not paint. Enter the coverage from the paint label and the number of coats you plan to apply. The coverage value applies to one coat.
The formula
A worked example
A 12 × 10 ft room with 8 ft walls has 352 sq ft of wall. Subtracting 32 sq ft of openings gives 320 sq ft. Two coats at 350 sq ft per gallon require 1.83 gallons, or 2 one-gallon cans.
Assumptions & limitations
Excludes ceilings and trim. Texture, surface porosity, color changes, and application technique affect coverage. One-gallon cans are assumed for the purchase estimate; primer is separate.
Common questions
Should I include the ceiling?
No. Calculate the ceiling area separately and use its own paint coverage and coat count.
Why is my paint label different?
Coverage is product-specific and often a range. Use the lower coverage value when you need a more conservative estimate.
Supporting reference
The method above states this calculator’s assumptions. This reference provides related definitions and technical context.
Behr: how much exterior paint to buy ↗Formula and worked example checked against automated test cases. About our methodology.