Enter your room size and this calculator subtracts the doors and windows, applies your coats, and tells you the gallons to buy — no halfway-through-the-room paint run.
Based on standard coverage of 350 sq ft per gallon, a door at 21 sq ft and a window at 15 sq ft. Rough or unpainted walls drink more — add about 10%.
Wall area is the room's perimeter times the ceiling height: 2 × (length + width) × height. From that we subtract the openings — about 21 sq ft for a standard door and 15 sq ft for an average window — then divide by the paint's coverage (350 sq ft per gallon is the standard figure on most cans) and multiply by your coats.
Two coats is the honest default for most rooms: one coat rarely covers evenly, especially over a darker colour. Painting a ceiling too? Its area is simply length × width — add it as a separate calculation.
| Room | Wall area | 1 coat | 2 coats |
|---|---|---|---|
| 10 × 10 ft room | 269 sq ft of wall | 0.77 gal | 1.54 gal |
| 10 × 12 ft room | 301 sq ft of wall | 0.86 gal | 1.72 gal |
| 12 × 12 ft room | 333 sq ft of wall | 0.95 gal | 1.90 gal |
| 12 × 15 ft room | 381 sq ft of wall | 1.09 gal | 2.18 gal |
| 15 × 20 ft room | 509 sq ft of wall | 1.45 gal | 2.91 gal |
Working out the room itself first? The square feet calculator handles the floor area, and our guide to how far 5 gallons of paint goes covers big jobs.
Walls come to about 333 sq ft after one door and two windows: roughly 1 gallon for one coat, 2 gallons for two coats (1.9 exactly, so two cans).
Primer is separate — it seals the surface but does not give final colour or finish. On bare drywall or big colour changes, budget one primer coat plus two paint coats.
That is for smooth, sealed walls. Rough texture, bare plaster and dark-to-light colour changes all reduce real coverage; 350 is the safe planning number.
Up, always. Leftover paint in a sealed can keeps for touch-ups; running out mid-wall risks a visible colour mismatch between batches.