Paint Calculator
Buying paint is a guessing game until you do the sum. It comes down to wall area × number of coats, divided by the coverage on the tin (usually about 10-12 m² per litre per coat). Enter your room and the calculator does it; the notes below cover what to add and subtract.
The formula
Wall area = perimeter × height = 2 × (length + width) × ceiling height.
Paint (litres) = wall area × coats ÷ coverage per litre.
A 4 m × 3 m room with a 2.4 m ceiling has 2 × 7 × 2.4 = 33.6 m² of wall. Two coats at 10 m²/L needs 33.6 × 2 ÷ 10 = 6.7 litres.
What to adjust
- Doors and windows: subtract about 1.5 m² per standard door and 1 m² per average window.
- Coverage varies: matt emulsion covers more than a hard-wearing satinwood; check the tin.
- Dark over light, or bare plaster: expect a third coat, or a separate primer.
- Buy about 10% extra for touch-ups and to keep a colour match.
Ceilings and trim
This calculator does walls. For the ceiling, add length × width to the area before applying coats. Trim (skirting, architraves) uses a different, much smaller quantity – usually one small tin does a room. To get room dimensions from an imperial plan, convert with feet to metres; for tiling the same room, the tile calculator works the same way with a waste allowance.
FAQ
How much paint do I need for a 4x3 metre room?
With a 2.4 m ceiling the walls are about 33.6 m². Two coats at 10 m²/L coverage need roughly 6.7 litres - so a 5 L tin plus a 2.5 L tin, or a single 10 L.
How many coats of paint do I need?
Two is standard for interior walls. Add a third when going light over dark or using budget paint. Bare plaster or drywall needs primer first.
Does the calculator include the ceiling?
No, walls only. Add length × width for the ceiling before applying the coats and coverage.
How much does a litre of paint cover?
Typically 10-12 m² per coat for interior emulsion. Textured surfaces and bare plaster soak up more, so coverage drops.