Measure the surfaces
Walls: add up the length of the walls (the perimeter) and multiply by the height, then take off the doors and windows. A 4 × 3.5 m room with a 2.4 m ceiling has 15 × 2.4 = 36 m² of wall; take off a door (about 1.6 m²) and a window (about 1.4 m²) and that's 33 m². Ceiling: length × width, 14 m².
Coverage by paint type
| Paint | Covers per litre | Use |
|---|---|---|
| Matt emulsion | about 12 m² | Interior walls and ceilings |
| Silk or satin emulsion | about 11 m² | Kitchens, bathrooms, hallways |
| Gloss or satinwood | about 14 m² | Doors, skirting, metal |
| Undercoat or primer | 10 to 12 m² | Bare wood, metal, new plaster |
| Masonry paint | about 6 m² | Brick, render, pebbledash |
These are typical figures. Rough or porous surfaces take more, and the tin always gives the manufacturer's own coverage, which is the one to use.
Worked example
| Surface | Working | Litres | +10% | Tins |
|---|---|---|---|---|
| Walls (one colour) | 33 m² × 2 coats ÷ 12 | 5.50 | 6.05 | 1 × 5 L + 2 × 1 L |
| Ceiling (white) | 14 m² × 2 coats ÷ 12 | 2.33 | 2.57 | 1 × 2.5 L + 1 × 1 L |
Mixing tin sizes keeps the leftover small. If the ceiling is in good condition and the same white, one 2.5 L tin may just cover it; the 10% allowance is for touch-ups and a porous surface.
New plaster and colour changes
- New plaster needs a mist coat (emulsion thinned with water, or a dedicated primer) first, which adds about one coat's worth.
- Going from dark to light, or using a strong colour, often needs a third coat.
- Masonry paint on rough render can use double the paint of smooth brick.