How Much Paint You Need — Gallons by Coat

Interior wall paint is a coverage problem. First find the wall area: multiply the room perimeter — twice the length plus twice the width — by the ceiling height. That gives the gross area of the four walls. Then subtract the openings you will not paint: a standard door is about 21 sq ft and an average window about 15 sq ft, so the calculator deducts those once per door and window you enter.

Divide the net wall area by the coverage rate on the can. Sherwin-Williams and Behr both put one gallon of interior latex at roughly 350 to 400 sq ft per coat on primed drywall, and less on bare or heavily textured surfaces. Multiply by the number of coats — two is normal for a color change — and the result is the gallons you need. Because you cannot buy a partial can, the count always rounds up.

The tool works in both unit systems. Enter feet and get gallons; switch to metric and the same math reports the equivalent in liters, since paint outside the US is sold in liter cans.