Paint calculator - how much paint for a room

Litres = area [m²] × coats / coverage [m²/l] × (1 + extra). Wall area is the room perimeter times the height: 2 × (length + width) × height, minus windows and doors.

Example: a 4 × 3 m room with 2.6 m walls has 2 × (4 + 3) × 2.6 = 36.4 m² of wall. Subtract a 1.4 × 1.5 m window (2.1 m²) and a 0.9 × 2 m door (1.8 m²) and you get 32.5 m². With 2 coats, 10 m²/l coverage and 10% extra: 32.5 × 2 / 10 × 1.1 = 7.15 litres - two 5-litre tins or three 2.5-litre tins. Adding the 12 m² ceiling brings it to 9.79 litres.

The coverage on the tin applies to smooth, primed walls. On fresh plaster, unprimed plasterboard, textured walls or when covering a dark colour, you can use 20-40% more paint - enter a lower coverage or a bigger margin.

Most walls need 2 coats. A third coat helps when going from dark to light, with strong colours or over stains. New plaster should be primed first - count primer separately.

An extra 10% covers paint left in the roller and tray plus touch-ups. If your colour is tinted in store, buy it all at once - a second batch may differ slightly in shade.

The calculator works in metres, litres and 2.5, 5 or 10 litre tins. If your paint is sold in US gallons, convert: 1 US gallon = 3.785 litres, so 7.15 litres is about 1.9 gallons - two 1-gallon cans. Coverage printed as sq ft per gallon converts to m²/l by dividing by about 40.7: 400 sq ft/gal ≈ 9.8 m²/l, 350 sq ft/gal ≈ 8.6 m²/l.

For rooms that are not a simple box - a sloped attic wall, a feature wall in a different colour, a stairwell - switch to Individual walls and enter each wall as width × height; for a sloping wall use the average height. Enter the ceiling area separately if it is not a rectangle. Use the square metre calculator for triangles and trapezoids, then type the result as one wall.