How much paint do I need for a room?
A 12 × 12 ft bedroom with 8 ft ceilings needs about 2.2 gallons for two coats on the walls. Step-by-step method, a chart by room size and the mistakes to avoid.
Key points
- Formula: wall area (sq ft) × coats ÷ coverage (sq ft per gallon), plus about 10%.
- Sherwin-Williams: one gallon typically covers 350–400 sq ft per coat on a smooth, prepared surface.
- 12 × 12 ft room, 8 ft ceilings, one door and one window: 349 sq ft of wall, about 2.2 gallons for two coats at 350 sq ft/gal.
Step by step
- Measure the length and width of the room and the ceiling height.
- Wall area = (length + width) × 2 × height.
- Subtract openings: about 20 sq ft per standard door and 15 sq ft per average window (measure large windows and patio doors).
- Multiply by the number of coats (usually 2).
- Divide by the coverage on the can, in square feet per gallon per coat.
- Add about 10% for touch-ups and waste.
Chart by room size
Assumptions: 8 ft ceilings, one door, two coats, 350 sq ft per gallon, 10% extra.
| Room | Size | Wall area to paint | Paint for walls | Ceiling (2 coats) |
|---|---|---|---|---|
| Small bedroom | 10 × 10 ft | 285 sq ft (1 window) | 1.8 gal | 0.6 gal |
| Bedroom | 12 × 12 ft | 349 sq ft (1 window) | 2.2 gal | 0.9 gal |
| Living room | 14 × 18 ft | 447 sq ft (3 windows) | 2.8 gal | 1.6 gal |
Use our paint calculator for your own dimensions; it also suggests a mix of gallons and quarts.
Coverage is the number that matters
The coverage rate is on every can. Sherwin-Williams states that one gallon typically covers 350–400 sq ft per coat on a prepared, smooth surface. Expect less on:
- new drywall or bare plaster, which soaks up paint (use a primer first);
- textured surfaces like knockdown, popcorn or stucco;
- dramatic colour changes, particularly light over dark, which may need a third coat.
In those cases, plan with about 300 sq ft per gallon.
Common mistakes
- Forgetting the ceiling and trim. Ceilings usually take a flat ceiling paint; trim and doors often a semi-gloss.
- Buying exactly the calculated amount and then needing one more can from a different batch with a slightly different shade.
- Over-subtracting openings in small rooms. A small window barely changes the result.
Do I need primer?
Use primer on new drywall, patched areas, stains, glossy surfaces or when making a big colour change. Calculate it the same way, usually with one coat and the coverage stated on the primer can.