Paint Calculator

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.