About this tool
Paint Calculator is a free, in-browser tool that estimates how much paint a room needs. Enter the total wall perimeter and wall height, how many doors and windows to subtract, the number of coats and the paint's coverage rate, and it returns gallons and liters to buy. Nothing is uploaded — everything is computed on your device.
The net paintable area is perimeter × wall height − doors × 21 ft² − windows × 15 ft², floored at zero, using 21 ft² for a standard door and 15 ft² for a standard window. Gallons = ceil(net area × coats ÷ coverage), rounded up because paint is sold in whole cans; liters are gallons × 3.78541. Coverage defaults to 350 ft² per gallon, a common figure for one coat on primed drywall.
Use it before a room repaint: measure the perimeter with a tape, set two coats for good coverage over a color change, and adjust the coverage rate to match the number printed on your paint can.