Enter the room size, doors, windows and coats to see exactly how much paint to buy.
How much paint do I need?
Gallons = (wall area − doors × 20 − windows × 15) × coats ÷ coverage
Wall area is the room's perimeter times its height: a 12 × 14 ft room with 8 ft walls has 2 × (12 + 14) × 8 = 416 sq ft. Take off one door (20 sq ft) and two windows (30 sq ft) to get 366 sq ft. Two coats at 350 sq ft per gallon need about 2.1 gallons, so buy 2 gallons and a quart.
Coverage
Most interior paints cover about 350 to 400 sq ft per gallon on smooth, primed walls. Use a lower number for textured walls, bare drywall, or a big color change; check the can for your paint's rating.
Tips
- Two coats are usually needed for even color, especially when changing colors.
- Prime first over new drywall, patches, stains or dark colors.
- Buy it all at once so the color matches; keep a little for touch-ups.
- Ceilings are usually painted with flat ceiling paint; choose "Walls and ceiling" to include them.
How we checked this
Before every update, this calculator is run in a real browser with these known answers, and it must match all of them:
- 12 × 14 room, 8 ft walls, 1 door, 2 windows, 2 coats: 2.09 gallons ✓
Last checked October 26, 2026. Spotted something wrong? The math is done in your browser with the formula shown on this page, so you can check any result by hand.
Questions people ask
How much paint for a 12x12 room?
With 8 ft walls, the walls are 384 sq ft. After a door and two windows, two coats need about 2 gallons.
How many square feet does a gallon of paint cover?
About 350 to 400 sq ft for one coat on smooth walls.
Do I need two coats of paint?
Usually, yes. One coat can look patchy, and two give an even color and better durability.
How much paint does a door take?
One side of a standard door is about 20 sq ft, so a quart covers a door with two coats.