Paint Calculator
Work out how much paint you need for a room's walls and ceiling, minus doors and windows.
About the Paint Calculator
Wall area is the room's perimeter times its height. Subtract about 1.9 m² (21 ft²) for each door and 1.5 m² (15 ft²) for each window, add the ceiling if needed, multiply by the number of coats and divide by the paint's coverage, printed on the tin. Two coats are usual, and a third may be needed for strong colour changes or fresh plaster.
The formula
Paint = (2 × (L + W) × H − openings + ceiling) × coats ÷ coverage
Worked example
Example: Paint needed = 2.5 gal
- Inputs: Units: Imperial (ft, in); Room length: 17 ft; Room width: 13 ft; Wall height: 9 ft; Doors: 1; Windows: 2; Paint the ceiling too: No; Coats: 2; Coverage per litre / gallon: 390.
- Formula: Paint = (2 × (L + W) × H − openings + ceiling) × coats ÷ coverage
- Result: Paint needed = 2.5 gal
- Wall area: 540 ft²
- Doors and windows removed: 52.7 ft²
Questions people ask
How much does a litre of paint cover?
Usually 10–12 m² per coat on smooth, primed walls; textured or porous surfaces take more.
Should I buy extra?
A little: keep some for touch-ups from the same batch.