How the paint calculator works
Wall area x coats, divided by coverage per gallon. For a room the calculator works out wall area from length, width and ceiling height, subtracts 21 sq ft per door and 15 sq ft per window, then divides by your coverage rate — about 350 sq ft per gallon on smooth, primed walls.
Two coats is the standard for a durable, even finish. Coverage drops on rough, textured or unprimed walls, so budget primer for bare drywall or dark-to-light colour changes.
Want the full installed price with labour? Use the interior painting cost calculator.