How the paint calculator works
The tool measures the four walls as the room perimeter times the ceiling height, then removes a typical 21 sq ft for each door and 15 sq ft for each window. It multiplies the remaining paintable area by your number of coats and divides by the coverage per gallon to estimate how much paint to buy.
Getting a better estimate
Coverage varies with the surface and the paint. New drywall, bare wood and bold color changes soak up more paint, so lean toward two coats and a primer. Check the can for its stated coverage and update the coverage field, and always round up to the nearest gallon so you do not run out mid-wall.