Key takeaways
- Wall area = perimeter × ceiling height — that's 2 × (length + width) × height.
- A gallon of interior paint covers about 350–400 sq ft in one coat on smooth drywall.
- Subtract ~10% for doors and windows, then multiply by the number of coats.
- Round up to whole gallons — primer and porous or textured surfaces cover less.
How to calculate paint
A gallon of interior paint covers about 350–400 square feet in one coat. To size a job, find the wall area, take out the doors and windows, multiply by the number of coats, then divide by that coverage and round up to whole gallons.
Coverage is the spread rate printed on the can. Smooth, sealed drywall is at the high end; new or primed drywall, and rough or porous surfaces, fall lower, so the same room can need more paint than the math first suggests.
Worked example: a 12 × 12 room
For a 12 ft × 12 ft room with 9 ft ceilings, two coats, and 10% openings at 350 sq ft per gallon: wall area is 2 × (12 + 12) × 9 = 432 sq ft. Take out 10% for doors and windows and the paintable area is 432 × 0.90 = 389 sq ft. Two coats make 389 × 2 = 778 sq ft, and 778 ÷ 350 = 2.2 gallons — so you buy 3 one-gallon cans, with a little left for touch-ups.
Coverage by surface (sq ft per gallon, one coat)
| Surface | Coverage per gallon |
|---|---|
| Smooth drywall / sealed | 350–400 sq ft |
| New / primed drywall (primer coat) | 200–300 sq ft |
| Textured or porous | 250–300 sq ft |
| Rough exterior / masonry | 200–250 sq ft |
Coats and surface prep
Two coats are standard for even color and durability, and going dark-to-light or using a vivid color often needs an extra one. New drywall should be primed first, and primer covers less than topcoat, so figure it as its own coat. To estimate the drywall itself before you paint, use the drywall calculator.
Frequently asked questions
How much paint do I need for a room?
Wall area minus ~10% for openings, times coats, divided by ~350 sq ft/gal. A 12×12 room with 9 ft walls and two coats ≈ 3 gallons.
How much paint for a 12×12 room?
A 12×12 room with 9 ft ceilings has 432 sq ft of wall; minus 10% and times two coats is 778 sq ft, ÷ 350 ≈ 2.2 gallons — buy 3 cans.
How many square feet does a gallon cover?
About 350–400 sq ft in one coat on smooth primed drywall; less on rough, porous, or freshly primed surfaces.
Do I need two coats?
Usually yes, for even color and durability. This tool defaults to two, which doubles the paintable area before dividing by coverage.
Should I subtract doors and windows?
Yes — they aren't painted. About 10% is typical for one door and a window or two; raise it for rooms with large openings.
How much for two coats on 400 sq ft?
400 × 2 = 800 sq ft ÷ 350 ≈ 2.3 gallons, so buy 3 cans. One gallon only covers a single coat over a similar color.
Coverage follows standard manufacturer data — paint makers such as Sherwin-Williams publish a spread rate of about 350–400 sq ft per gallon for one coat on smooth, sealed surfaces.
Last reviewed June 2026