One gallon of interior paint covers about 350 to 400 square feet per coat. To find how much you need, multiply the room’s perimeter by the wall height, subtract about 20 sq ft per door and 15 per window, multiply by 2 coats and divide by 350. A 12×12 bedroom with 8 ft ceilings needs about 2 gallons.
Buying paint is easy to get wrong in both directions. Too little and you’re back at the store mid-project, hoping the new can matches. Too much and you’re storing half-empty gallons for years. Ten minutes with a tape measure fixes both.
The paint formula
Paintable area = wall area − (20 × doors) − (15 × windows)
Gallons = paintable area × coats ÷ 350
We use 350 sq ft per gallon, the low end of the 350–400 range Sherwin-Williams gives, so you’re less likely to run short.
How much paint you need by room size
Walls only, two coats, 350 sq ft per gallon:
| Room | Paintable area | Gallons | What to buy |
|---|---|---|---|
| Small bathroom (5×8 ft, 8 ft walls) | 188 sq ft | 1.07 | 1 gal + 1 qt |
| Bedroom (10×10 ft, 8 ft walls) | 285 sq ft | 1.63 | 2 gal |
| Bedroom (12×12 ft, 8 ft walls) | 349 sq ft | 1.99 | 2 gal |
| Primary bedroom (14×16 ft, 9 ft walls) | 490 sq ft | 2.80 | 3 gal |
| Living room (15×20 ft, 9 ft walls) | 560 sq ft | 3.20 | 3 gal + 1 qt |
| Large family room (20×20 ft, 10 ft walls) | 715 sq ft | 4.09 | 4 gal + 1 qt |
Step-by-step example
Ashley is painting a 14 × 12 ft bedroom in Charlotte with 9 ft walls, one door and two windows.
- Perimeter: 2 × (14 + 12) = 52 ft
- Wall area: 52 × 9 = 468 sq ft
- Minus openings: 468 − 20 − 30 = 418 sq ft
- Two coats: 418 × 2 = 836 sq ft
- Gallons: 836 ÷ 350 = 2.39 gallons
She buys 2 gallons plus 2 quarts, about 2.5 gallons, which leaves a little for touch-ups.
What changes how far paint goes
- Texture: textured walls, brick and stucco have more surface area and soak up more paint.
- Bare drywall: new drywall absorbs paint unevenly. Prime it first.
- Big color changes: going from dark to light usually needs a tinted primer or an extra coat.
- Application: rollers with thick nap and sprayers both use more paint than a careful brush-and-roll.
Ceilings, trim and primer
Ceilings: length × width. A 12×12 ceiling is 144 sq ft, so one gallon handles two coats. Choose “Paint the ceiling” in our paint calculator to include it.
Primer: a gallon of primer covers about 200 to 300 sq ft, so budget more primer than you might expect for new drywall.
Trim and doors: trim takes much less paint than walls. For one room, a quart of trim paint is often enough, but check the can’s coverage for your surface.
Gallons, quarts or 5-gallon buckets?
A gallon is 4 quarts. Quarts cost more per ounce, so once you need more than half a gallon extra, a full gallon is usually the better buy. If you’re painting several rooms the same color, a 5-gallon bucket is cheaper per gallon and avoids color differences between cans. For outdoor projects measured in volume, like mulch or gravel, use our cubic yard calculator.
Frequently asked questions
How many square feet does a gallon of paint cover?
About 350 to 400 square feet per coat on smooth, previously painted walls. Primer covers less, about 200 to 300 square feet per gallon.
How much paint do I need for a 12×12 room?
With 8 ft ceilings, one door and one window, the walls are about 350 sq ft. Two coats need about 2 gallons.
Do I need two coats of paint?
Usually. Two coats give even color and a more durable finish, and they are almost always needed when you change color.
Is it cheaper to buy quarts or gallons?
Gallons are cheaper per ounce. If you need more than about half a gallon extra, buy another gallon instead of several quarts.
How much paint do I need for a ceiling?
Multiply the room length by width for the ceiling area. A 12×12 ceiling is 144 sq ft, so one gallon covers two coats.
Sources
- Paint Calculator: How Much Paint Do I Need? — Sherwin-Williams, coverage per gallon of paint and primer