Paint needed = area × coats ÷ spread rate. The spread rate is the part most estimates get wrong — it varies by roughly two to one between a smooth interior wall and a roof coating, and the tin's stated figure assumes a sealed, even substrate.
These are the rates Surfacely prices with. A porous, previously unpainted or heavily textured surface takes the low end of the range; a sealed, smooth, previously painted one takes the high end.
| Paint type | Typical spread rate | Range | Imperial |
|---|---|---|---|
| Interior acrylic / latex / emulsion | 15 m²/L | 14 – 16 m²/L | 611 ft²/gal |
| Interior enamel / gloss | 15 m²/L | 14 – 16 m²/L | 611 ft²/gal |
| Exterior masonry / weather coating | 13 m²/L | 12 – 14 m²/L | 530 ft²/gal |
| Roof coating | 8 m²/L | 6 – 10 m²/L | 326 ft²/gal |
| Epoxy floor | 7 m²/L | 6 – 8 m²/L | 285 ft²/gal |
| Primer / sealer | 12 m²/L | 10 – 14 m²/L | 489 ft²/gal |
A room with 42 m² of wall, two coats of interior acrylic at 15 m²/L: 42 × 2 ÷ 15 = 5.6 L. Bought in 10 L tins that is one tin, with enough left for touch-ups. In imperial: 452 ft² × 2 ÷ 611 ft²/gal ≈ 1.5 gallons.
Openings are not deducted unless they are large. A standard door and a window in a bedroom wall are inside the wastage the spread rate already assumes; a full glazed wall is not.