Paint calculator
Measure the room, say how many coats, and check the coverage figure on the tin. Doors and windows are deducted, and the ceiling is optional since it usually takes a different paint.
- 5 litre tins to buy
- 2
- Surface to paint, m²
- 33.90
- Wall area after openings, m²
- 33.90
- Deducted for doors and windows, m²
- 4.60
- Floor area, m²
- 14.40
- Room perimeter, m
- 15.40
- Cost of the tins
- —
The formula
Wall area = 2 × (length + width) × height, less 1.8 m² per door and 1.4 m² per window. Paint = area × coats ÷ coverage. The ceiling adds length × width when included.
Worked example
A 4.5 by 3.2 m room with 2.5 m walls, one door and two windows: 33.9 m² of wall. Two coats at 11 m² per litre needs 6.16 litres — two 5 litre tins.
How to use it
- 1Take the coverage from the tin, not from a general rule: it ranges from 6 to 16 m² per litre depending on the paint.
- 2Allow extra for bare plaster, which drinks the first coat, and for a dark-to-light colour change, which may need three coats.
- 3Buy whole tins from the same batch. Mixing batches of the same colour can show on a large wall.
Questions people ask
Why does the first coat use so much more?
Because bare or porous surfaces absorb it. A mist coat thinned with water on new plaster seals the surface and saves paint overall.
Should I measure the actual doors and windows?
For a precise job, yes — this uses 1.8 m² and 1.4 m², typical for a standard door and window. Large glazing changes the answer enough to matter.
Last reviewed: 2026-10-20