How to calculate paint
Start with the total wall area — the room's perimeter times the wall height — then subtract the doors and windows you won't paint. Multiply by the number of coats and divide by the paint's coverage rate:
Gallons = (Wall area − openings) × coats ÷ coverage
Wall area is 2 × (length + width) × height. For a 12 × 12 room with 8-ft walls that's 2 × 24 × 8 = 384 square feet. Subtract one door (≈21 sq ft) and two windows (≈15 sq ft each) to get about 333 paintable square feet. Two coats over that, at 350 sq ft per gallon, is roughly 1.9 gallons — so buy 2.
What to subtract for openings
- Standard door: about 21 sq ft (3 ft × 7 ft).
- Average window: about 15 sq ft.
These are typical sizes the calculator uses. Skipping them and painting the full walls just leaves you with a little extra, which is rarely a bad thing.
Coverage and coats
A gallon covers roughly 350–400 square feet per coat on smooth, primed walls — less on rough or porous surfaces. Plan on two coats for an even finish, especially when changing color or painting bare drywall. Primer is counted separately.
Frequently asked questions
How much paint do I need for a room?
Find the wall area — the room perimeter times the ceiling height — then subtract about 21 square feet per door and 15 per window. Multiply by the number of coats and divide by the paint coverage (about 350 square feet per gallon). A 12 × 12 room with 8-ft walls needs roughly 2 gallons for two coats.
How many square feet does a gallon of paint cover?
About 350 to 400 square feet per coat on a smooth, primed surface. Rough, porous, or previously unpainted walls absorb more and cover less, and dark-to-light color changes usually need an extra coat. The calculator defaults to 350 for a safe estimate.
How many coats of paint do I need?
Two coats is standard for an even, durable finish, and is almost always needed when changing color or painting over bare drywall. One coat may be enough when repainting a similar color over a sound, primed surface. Primer is separate and recommended on new or patched walls.
Should I include the ceiling?
This calculator estimates wall paint only. For a ceiling, calculate its area separately (length × width) and divide by the coverage rate. Ceilings often use a dedicated flat ceiling paint rather than wall paint.
Disclaimer: Coverage varies with surface texture, color, and application. Results are estimates — buy a little extra so you don't run out mid-wall, and keep some for touch-ups.