The one number that drives everything: coverage rate
Every paint calculation comes down to how much area a fixed amount of paint covers, called the spread rate or coverage rate. It's printed on the can, and as a general baseline, interior paint covers roughly 350–400 square feet per US gallon (about 32–37 m²) in a single coat. In metric terms that works out to somewhere around 8.5–10 square meters per liter, though labels in different regions quote anything from about 8 to 12 m²/L depending on the product.
| Coverage (ft² per US gallon) | Same in m² per gallon | Same in m² per liter |
|---|
| 250 | 23.2 | 6.1 |
| 300 | 27.9 | 7.4 |
| 350 | 32.5 | 8.6 |
| 400 | 37.2 | 9.8 |
If you're buying in imperial (UK) gallons rather than US gallons, the numbers shift: a UK gallon is about 4.55 liters, versus 3.79 liters for a US gallon, so the same 350 ft² coverage per gallon works out to a lower m² per liter figure.
The formula
Paint needed = (Wall area − Openings) × Coats ÷ Coverage rate
Then add a waste allowance of about 10% (professionals often use 10–15%) for paint left in the can and tray, drips, and touch-ups, and round up to the next can size.
Worked example: a 12 × 14 ft room (imperial)
- Wall area: perimeter 2 × (12 + 14) = 52 ft, times an 8 ft ceiling height = 416 sq ft.
- Subtract openings: one standard door (about 21 sq ft) and two standard windows (about 15 sq ft each) = 51 sq ft. Net paintable area: 365 sq ft.
- Two coats: 365 × 2 = 730 sq ft of coverage to buy.
- At 350 sq ft/gallon: 730 ÷ 350 ≈ 2.09 gallons. Add 10% waste: about 2.3 gallons — so plan on the next size up rather than exactly two gallons.
The same job in metric
- Room: about 3.6 m × 4.2 m with a 2.4 m ceiling. Perimeter 15.6 m × 2.4 m = 37.4 m² of wall.
- Subtract openings: a door (~1.9 m²) and two windows (~1.4 m² each) = about 4.7 m². Net: about 32.7 m².
- Two coats: about 65.5 m².
- At 9 m² per liter: 65.5 ÷ 9 ≈ 7.3 liters. Add 10% waste: about 8 liters.
The two calculations agree, which is the point: the method is identical, only the units change. To convert any area or volume yourself, use the Area and Volume categories on the UnitDial converter.
Coverage varies by surface
The 350–400 figure assumes a smooth, previously painted wall. Rougher or more absorbent surfaces use noticeably more paint:
| Surface | Approx. coverage (ft² per gallon) |
|---|
| Smooth drywall | ~400 |
| Textured walls or ceilings | ~350 |
| Wood siding | ~275 |
| Brick | ~200 |
| Stucco | ~175 |
Porous materials such as unsealed block, old weathered wood, or brick can soak up 15–25% more paint on the first coat, which is why priming them first usually saves money overall.
Common mistakes
- Forgetting the ceiling. It's a separate calculation — length times width — and is usually painted in a flat finish with its own coverage rate.
- Assuming one coat. One coat is usually enough only when repainting a similar color over a sound surface; big color changes, stains, or bare drywall typically need two or more.
- Subtracting too much for openings. Standard door and window allowances are small relative to a whole room; deducting generously can leave you a few square feet short.
- Not rounding up. A second trip for half a gallon is annoying, and matching the color across separate batches is harder than buying a little extra now.
- Trim and primer. Trim in semi-gloss covers less area per gallon than flat wall paint, and primer has its own coverage rate — calculate each separately.