How Much Paint Do You Need? Room Formula With Example
Calculate paint for any room: wall area, minus doors and windows, times coats, divided by coverage. Worked 4 × 5 m example.
The formula
Paint needed = (wall area − door and window area) × number of coats ÷ coverage per litre. Wall area is the room perimeter times the wall height.
Worked example
Room 4 m × 5 m, ceiling height 2.5 m. Perimeter = 2 × (4 + 5) = 18 m. Wall area = 18 × 2.5 = 45 m². Subtract one door (2 m²) and one window (1.5 m²) to get 41.5 m². Two coats = 83 m². If the tin says 10 m² per litre, you need 8.3 litres; add about 10% for waste and touch-ups to get roughly 9.1 litres.
Tips
Check the coverage printed on the tin because it varies by product and surface. Porous or very dark walls may need a primer or a third coat. Buy all paint for one wall at the same time so the colour matches.
Use the calculator
The paint calculator handles several walls and openings, metric or US units, and the number of coats.