How to use the paint calculator
Measure the length and width of the room along the floor, and the wall height from floor to ceiling. Count the doors and windows. Choose the number of coats, then check the coverage on your paint can and enter it if it differs from 350 sq ft per gallon. The calculator shows the gallons you need and how to buy them in gallons and quarts.
How paint quantity is calculated
- Wall area = 2 × (length + width) × height
- Openings = doors × 21 sq ft + windows × 15 sq ft
- Area to paint = wall area − openings (+ length × width if you include the ceiling)
- Gallons = area to paint × coats ÷ coverage per gallon
A standard interior door is 3 × 7 ft, which is where the 21 sq ft comes from. Windows vary more, so 15 sq ft is an average for a 3 × 5 ft window. If you have a sliding door or a large picture window, measure it and treat it as two openings.
When the result lands between whole gallons, the calculator adds quarts. Three or more quarts round up to another gallon, since that usually costs about the same.
Coverage: why the can says 250 to 400 sq ft
Paint makers print a range because the surface changes how far a gallon goes:
| Surface | Expect per gallon |
|---|---|
| Smooth, previously painted drywall | 350–400 sq ft |
| New drywall, primed | 300–350 sq ft |
| Light texture (orange peel) | 300–350 sq ft |
| Heavy texture, popcorn ceiling | 200–300 sq ft |
| Bare wood, masonry, brick | 150–250 sq ft |
Paint for common rooms
Walls only, two coats, one door and one window, 350 sq ft per gallon:
| Room | Size | Wall area | Gallons | Buy |
|---|---|---|---|---|
| Small bedroom | 10 × 10 ft, 8 ft walls | 284 sq ft | 1.6 | 2 gal |
| Bedroom | 12 × 12 ft, 8 ft walls | 348 sq ft | 2 | 2 gal |
| Primary bedroom | 12 × 14 ft, 8 ft walls | 380 sq ft | 2.2 | 2 gal + 1 qt |
| Living room | 14 × 16 ft, 9 ft walls | 504 sq ft | 2.9 | 3 gal |
| Bathroom | 5 × 8 ft, 8 ft walls | 172 sq ft | 1 | 1 gal |
| Home office | 10 × 12 ft, 8 ft walls | 316 sq ft | 1.8 | 2 gal |
Worked example: a 12 × 14 ft living room
8 ft walls, two doors, two windows, two coats, walls only:
- Wall area: 2 × (12 + 14) × 8 = 416 sq ft.
- Openings: 2 × 21 + 2 × 15 = 72 sq ft.
- Area to paint: 416 − 72 = 344 sq ft.
- Paint: 344 × 2 ÷ 350 = 1.97 gallons.
- Buy 2 gal.
When you need primer
- New drywall or fresh patches, which soak up paint unevenly.
- Going from a dark color to a much lighter one.
- Glossy surfaces, stains, water marks or smoke damage.
- Bare wood, especially knotty pine or cedar that can bleed through.
Paint-and-primer-in-one products work well over sound, previously painted walls in a similar color. They do not replace a dedicated primer on the surfaces above.
Questions people ask
How many gallons of paint for a 12 × 12 room?
About 2 gallons for two coats on the walls with 8 ft ceilings, one door and one window. Buy 2 gal.
Is one coat enough?
Rarely. Two coats give even color and sheen, especially when changing colors. One coat can be enough when repainting the same color over a clean wall.
How much paint for trim?
Baseboards, door casings and window trim in an average room usually take less than a quart. Trim is normally a separate, glossier paint.