Short answer
A 12 ft × 12 ft room with 8 ft ceilings needs about 2 gallons for one coat and 3 gallons for two coats of wall paint, at the standard coverage of 350 sq ft per gallon. Walls only — the ceiling needs about 1 extra gallon.
The formula
Wall area = 2 × (length + width) × ceiling height.
Gallons = wall area × coats ÷ 350.
Always round up to whole gallons — paint is sold by the gallon, and running out mid-wall leaves visible lines.
Worked example
A room has 400 sq ft of wall area and you want 2 coats at 350 sq ft per gallon:
400 ÷ 350 = 1.14 gallons per coat.
1.14 × 2 coats = 2.29 gallons, so you buy 3 gallons.
In cans: at 100 sq ft per can, 400 ÷ 100 = 4 cans per coat, so 8 cans for two coats. Try it with your own numbers in the Paint Coverage Calculator or the Paint Cans Calculator.
Gallons by room size
8 ft ceilings, smooth primed walls, 350 sq ft per gallon, walls only. This table uses gross wall area, before subtracting doors and windows:
| Room | Wall area | 1 coat (buy) | 2 coats (buy) |
|---|---|---|---|
| 10 × 10 ft | 320 sq ft | 0.91 gal (1) | 1.83 gal (2) |
| 12 × 12 ft | 384 sq ft | 1.10 gal (2) | 2.19 gal (3) |
| 15 × 12 ft | 432 sq ft | 1.23 gal (2) | 2.47 gal (3) |
After subtracting doors and windows, you buy less. A standard door is about 21 sq ft and a standard window about 15 sq ft. For the 12 × 12 room: 384 − 21 − (2 × 15) = 333 sq ft. For 2 coats: 333 × 2 ÷ 350 = 1.90 gallons, so buy 2 gallons — not the gross-area 3. Use the gross table only when you will paint over doors and windows too, or want extra for touch-ups. Taller ceilings increase the wall area in proportion: a 9 ft ceiling needs about one-eighth more wall paint than an 8 ft ceiling for the same room.
Ceilings and primer
Ceiling area is length × width. A 12 × 12 ceiling is 144 sq ft, which is 0.41 gallons per coat — one gallon covers one coat. For ceilings, see the Ceiling Paint Calculator. For outside walls, see the Exterior Paint Calculator.
Primer is a separate coat, bought separately. It usually covers less than finish paint (typically 200–300 sq ft per gallon), so check the label on the can you buy.
Method and limits
This guide uses one stated coverage — 350 sq ft per gallon on smooth, primed walls — so every number above can be re-checked with the formula. Real coverage varies: rough or textured walls, bare drywall, and big colour changes all need more paint than smooth repainted walls, and the exact coverage is always the number on your own can. Results here are estimates for planning, not a purchase guarantee. How BuildCalc checks its calculators is explained on How Our Calculators Are Checked.
Try it with your own numbers
Enter your exact room size instead of guessing from the table: Paint Coverage Calculator, Paint Cans Calculator, or the general Paint Calculator. More paint tools are on the Paint Calculators page and in Construction calculators. More guides are on the BuildCalc Blog.
Results are estimates and general information only. Always check the paint can label for the exact coverage of the product you buy.