How to calculate paint
- Measure the room — length, width and wall height. For a single feature wall, multiply its width by its height and divide by the coverage per gallon.
- Subtract doors and windows you won't paint.
- Choose the number of coats. Two coats is standard for an even finish, especially when changing color.
- Check the coverage on the can — most interior paints cover 250–400 sq ft per gallon per coat.
Paint formula
Gallons = Paintable area × Coats ÷ Coverage per gallon
Worked example
A 12 × 12 ft bedroom with 8 ft walls has 384 sq ft of wall. Take away one door (20 sq ft) and two windows (30 sq ft) and you have 334 sq ft. Two coats at 350 sq ft per gallon need 1.91 gallons — buy 2 gallons.
Paint needed per room
8 ft walls, one door, two windows, 350 sq ft per gallon:
| Room | Wall area | Gallons (1 coat) | Gallons (2 coats) |
|---|---|---|---|
| 10 × 10 ft | 270 sq ft | 0.77 | 1.54 |
| 12 × 12 ft | 334 sq ft | 0.95 | 1.91 |
| 12 × 14 ft | 366 sq ft | 1.05 | 2.09 |
| 14 × 16 ft | 430 sq ft | 1.23 | 2.46 |
| 16 × 20 ft | 526 sq ft | 1.50 | 3.01 |
What affects coverage
- Surface texture: rough plaster, brick and textured drywall can use 20–50% more paint than smooth walls.
- New drywall absorbs paint unevenly — prime it first.
- Big color changes, especially light over dark or bright reds and yellows, may need a tinted primer or a third coat.
- Application: sprayers use noticeably more paint than rollers due to overspray.
Buying tips
- When you need four gallons or more, a 5-gallon bucket is usually cheaper per gallon and keeps the color consistent.
- For leftover amounts of a quart or two, buy quarts; three quarts or more, a gallon is usually better value.
- Keep a little leftover paint, labeled with the room and color, for touch-ups.
Frequently asked questions
How much paint do I need for a 12×12 room?
With 8 ft walls, one door and two windows, a 12 × 12 room has about 334 sq ft of wall. Two coats at 350 sq ft per gallon need about 1.91 gallons, so buy 2 gallons. Add about 0.8 gallon for two coats on the ceiling.
How many square feet does a gallon of paint cover?
Typically 250–400 sq ft per coat on smooth, primed walls. The exact figure is printed on the can; textured or porous surfaces get less coverage.
How many coats of paint do I need?
Two coats for most walls. One coat can work when repainting the same color over a sound surface; three may be needed for dramatic color changes without primer.
How do I calculate paint for a ceiling?
Multiply the room's length by its width for the ceiling area, multiply by the number of coats, and divide by the coverage per gallon. Tick “Paint the ceiling too” in the calculator.
Should I subtract doors and windows?
Yes. A standard door is about 20 sq ft and an average window about 15 sq ft. For rooms with very large windows or patio doors, measure them.