Key takeaways
- A gallon of interior paint covers about 350–400 square feet in one coat.
- Wall area = room perimeter × ceiling height, then subtract the openings.
- Subtract roughly 20 sq ft per door and 15 sq ft per window.
- Multiply by your number of coats, divide by coverage, and always round up.
Start with coverage
A gallon of interior paint covers about 350–400 square feet in one coat. That figure assumes smooth, sealed drywall in good condition. New, primed, or textured surfaces are thirstier, so coverage can fall well below 300 square feet per gallon. Knowing the surface you're painting is the difference between buying one gallon and buying two.
Everything else in the estimate is just figuring out how many square feet of wall you actually have to cover, then dividing by that coverage number. Get those two pieces right and the rest is arithmetic.
Measure the wall area
Add up the length of every wall to get the room's perimeter, then multiply by the ceiling height. That gives the gross wall area. From there, subtract the surfaces you won't paint: roughly 20 square feet for each standard door and 15 square feet for each window. The result is your net paintable area.
Then account for coats — most rooms need two for even color — and divide by the coverage of your chosen paint. Round the answer up to the next whole gallon, since you can't buy a partial can and a little extra is cheap insurance against running short mid-wall.
A worked example: a 12 × 12 room
Say you're repainting a 12 ft × 12 ft bedroom with 8 ft ceilings. The perimeter is 12 + 12 + 12 + 12 = 48 ft, and 48 × 8 = 384 sq ft of wall. Subtract one door (≈ 20 sq ft) and one window (≈ 15 sq ft), about 40 sq ft of openings rounded together, leaving 344 sq ft of paintable wall. Two coats make it 344 × 2 = 688 sq ft, and 688 ÷ 350 ≈ 2 gallons.
The paint calculator runs this instantly — enter the room dimensions, openings, and coats, and it returns the gallons for your surface type.
Coverage by surface
Coverage swings widely with the surface. Smooth, previously painted drywall gives you the full 400 square feet; raw or textured surfaces soak up far more. Match your estimate to what you're actually rolling over.
| Surface | Coverage (sq ft per gallon, one coat) |
|---|---|
| Smooth / sealed drywall | 350–400 |
| New / primed drywall (primer) | 200–300 |
| Textured or porous | 250–300 |
| Rough exterior / masonry | 200–250 |
Don't forget primer and prep
Bare or patched drywall needs a coat of primer before color, and primer follows the same area math at lower coverage. If you're finishing new walls, size the board and compound first with the drywall calculator, then come back here to estimate paint. Buy ceiling and trim paint separately — they use different finishes and their own area math.
Frequently asked questions
How much does a gallon of paint cover?
About 350–400 sq ft in one coat on smooth, sealed drywall. New or textured surfaces drop coverage to roughly 200–300 sq ft per gallon.
Do I subtract doors and windows from the wall area?
Yes — subtract about 20 sq ft per door and 15 sq ft per window from the total wall area before dividing by coverage.
How many coats of paint should I plan for?
Plan for two in most rooms. Multiply your net wall area by the number of coats before dividing by coverage per gallon.