A 12 × 14 ft bedroom with 8 ft walls, one door and two windows has 366 square feet of wall to paint. Two coats of interior latex at 350 ft² per gallon, plus 10 % for touch-ups and losses, come to 2.3 gallons — two gallons and two quarts. The calculator above works this out for any room or set of walls, with or without the ceiling, and tells you which cans to buy.
How the calculator works
Enter the room's length, width and wall height — or the length and height of the walls and how many there are — then the doors and windows. Choose the paint, check the coverage against your can, pick the surface and the number of coats, and add a coat of primer if the walls need one. The result is the paint in gallons and the containers to buy.
Area
The walls of a room are its perimeter times the height: 2 × (12 + 14) × 8 = 416 ft². Subtract about 20 ft² for each door and 15 ft² for each window — 366 ft² here. A ceiling adds length × width, 168 ft².
Coverage and coats
Gallons = area × coats × surface factor ÷ coverage per gallon. Interior latex usually covers 350–400 ft² per gallon on smooth drywall; textured walls take about 20 % more paint, brick and stucco 50 % more, primed walls about 10 % less.
| Room, 8 ft walls | Walls to paint | Two coats | With ceiling |
|---|---|---|---|
| 10 × 10 ft | 270 ft² | 1.7 gal | 2.3 gal |
| 12 × 14 ft | 366 ft² | 2.3 gal | 3.4 gal |
| 15 × 20 ft | 510 ft² | 3.2 gal | 5.1 gal |
The table assumes one door and two windows, 350 ft² per gallon and 10 % reserve.
Which cans to buy
Paint comes in quarts, gallons and 5-gallon buckets. A bucket pays off once you need more than four gallons of one color; under that, buy gallons and top up with a quart or two — three quarts cost about as much as a gallon. Mix all cans of one color together before you start so the shade is even.
Primer
New drywall, patched walls, bare wood and a big change of color need a coat of primer first. Primer covers about 300 ft² per gallon; the calculator adds it as a separate line.
Common mistakes
- One coat. Even "paint and primer in one" usually needs two coats to look even.
- Ignoring texture. Knockdown and orange-peel walls drink paint.
- Buying to the drop. Keep a quart for touch-ups.
For the walls themselves, use the Drywall Calculator.