Paint calculator

The formula, in plain words

1. 1Wall area per room = 2 × (length + width) × height. Add the rooms, then take off every door and window.
2. 2Ceiling area = length × width for each room whose ceiling is being painted.
3. 3Litres = area × coats ÷ coverage, then × (1 + wastage). Coverage is the spread rate on the tin, in m² per litre per coat.
4. 4Primer or undercoat is one coat over the walls and ceilings at the primer's own coverage.
5. 5Cans: from the sizes you can buy, the mix with the least left over, then the fewest cans. It never comes up short.

Worked example

One room 4 × 3.6 m with a 2.4 m ceiling, one 820 × 2040 mm door and one 1.2 × 1.2 m window. Two coats on walls and ceiling at 16 m² per litre, 10% wastage.

1. 1.Walls: 2 × (4 + 3.6) × 2.4 = 36.48 m², less the door (1.67 m²) and window (1.44 m²) = 33.37 m².
2. 2.Wall paint: 33.37 × 2 ÷ 16 × 1.1 = 4.59 L, bought as a 4 L and a 1 L can.
3. 3.Ceiling: 4 × 3.6 = 14.4 m² × 2 ÷ 16 × 1.1 = 1.98 L, one 2 L can.

Answer: About 6.6 litres: 4 L + 1 L of wall paint and 2 L of ceiling paint.

How to measure it

• Measure each room's length and width at floor level, and the wall height to the ceiling.
• Count the doors and windows and note their sizes. They come off the wall area.
• Read the spread rate on your paint's tin or data sheet and type it in if it differs from 16 m² per litre.
• Add a primer coat for new plaster, bare timber or patched walls.

Where the numbers come from

Every default in the calculator is a number a supplier or manufacturer publishes. We list the source next to it so you can check.

Frequently asked questions