Enter the room size and how many coats are planned, and get the paintable area and how much paint and primer to buy.
The calculator adds up the four walls (perimeter times ceiling height), subtracts a standard 21 sq ft for each door and 15 sq ft for each window, and optionally adds the ceiling. Paint needed is the paintable area multiplied by the number of coats, divided by the coverage printed on the tin — 350 sq ft per US gallon is a typical figure for smooth interior walls. Primer is worked out as a single coat over the same area. Rough, porous or heavily textured walls cover less per gallon, so lower the coverage figure to match the tin being used.
How much paint does one gallon cover? Most interior paints cover 300 to 400 sq ft per US gallon on smooth walls; the tin states the manufacturer's figure. This calculator defaults to 350 and lets the coverage be changed.
Should doors and windows be subtracted? Yes for a buying estimate — a standard door is about 21 sq ft and an average window about 15 sq ft. Trim and frames are usually painted in a separate trim paint, so they are left out here.