Enter your room size, ceiling height and how many coats, and get how many gallons of paint to buy — with doors and windows subtracted.
Paint is sold by how much area it covers. First find the wall area: add up the room's perimeter — twice the length plus twice the width — and multiply by the ceiling height. Then subtract the openings you won't paint: about 21 square feet per door and 15 square feet per window. Divide that paintable area by a gallon's coverage (about 300 square feet in real-world use) and multiply by the number of coats.
In real-world painting a gallon usually covers about 250–300 square feet in one coat, once you allow for cutting in, roller waste and wall texture. Manufacturers often print 350–400, but that's a smooth, primed best case. This calculator defaults to 300 square feet per gallon so you're not caught short — drop it to 250 for rough, porous or dark-to-light walls.
Two coats is the standard for an even, durable finish, especially over a new color or fresh drywall. One coat can be enough when you're refreshing a similar color over an already-painted wall. When in doubt, plan for two.
This is a planning aid for wall paint. It figures walls only (not the ceiling) and uses standard opening sizes — adjust if your doors or windows are unusually large. Coverage varies by paint and surface; buy a little extra for touch-ups.