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 |
| Floor & paving paint (single-pack) | 8 m²/L | 6 – 10 m²/L | 326 ft²/gal |
| Primer / sealer | 12 m²/L | 10 – 14 m²/L | 489 ft²/gal |
| Cabinet coating (2-pac / urethane) | 10 m²/L | 8 – 12 m²/L | 407 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 gal 1 qt 29 fl oz.
Take the doors and windows off the wall first: measure each one, or count them and use a standard size — about 18 ft² (1.7 m²) for a door and 15 ft² (1.4 m²) for a window. The calculator works on the area you enter, so enter the wall net of its openings. In Surfacely the doors and windows in an area come off its walls automatically, and the paint and labour follow the net area.