Paint is area work with two twists: you paint most walls twice, and the coverage on the can assumes a smooth, sealed surface. Get the wall area, take out the openings, multiply by coats, and divide by the coverage. Then round up — running short mid-wall risks a visible batch line.
Step 1: the wall area
Multiply the room perimeter by the ceiling height, then subtract the doors and windows. A 13 × 10 ft room with 8 ft ceilings has a (13+10) × 2 × 8 = 368 sq ft (34 m²) of wall. A standard door is about 17 sq ft (1.6 m²) and a window 10–20 sq ft, so take off roughly 32 sq ft — leaving about 336 sq ft.
Step 2: coats and coverage
Most latex paints cover 300–450 sq ft per gallon(8–11 m²/L) per coat on a sealed, smooth surface — the can states the figure. At two coats: 336 × 2 = 672 sq ft to cover, and at 350 sq ft/gal that is 672 ÷ 350 ≈ 1.9 gallons (about 7 L).
| Room (8 ft ceilings) | Wall area | Paint (2 coats) |
|---|---|---|
| 10 × 10 ft bedroom | ~290 sq ft | ~1.7 gal (6 L) |
| 13 × 10 ft room | ~336 sq ft | ~1.9 gal (7 L) |
| 15 × 15 ft living room | ~450 sq ft | ~2.6 gal (10 L) |
Paint Calculator
Enter the room or wall size, coats and your paint's coverage — it returns the gallons (litres) and a sensible mix of can sizes to buy.
How far does a gallon really go?
The can figure is for a sealed, smooth wall. Porous surfaces drink more: new plaster, bagged brick and textured finishes can drop below 250 sq ft/gal (6 m²/L) on the first coat — which is exactly why a primer or plaster sealer pays for itself, sealing the surface so the finish coats spread to their rated coverage.
How many coats?
Two finish coats is the standard for a solid, even color. Add a primer on new plaster or bare surfaces, and budget a third coat when going light over a dark color — or use a tinted undercoat to save a finish coat.
Buy it in one go
Always round up, and buy the whole job at once with matching batch numbers — separate batches can differ slightly in shade. Keep the labelled leftover for touch-ups: a patch from the same batch blends in far better than a later color match.