Paint Calculator
Work out how much paint you need for the walls and ceiling of a room.
About this tool
Works out the wall area from the room’s perimeter and height, subtracts about 1.9 m² for each door and 1.5 m² for each window, adds the ceiling if you choose, then divides by the coverage and multiplies by the number of coats. Check the coverage on your tin.
Buying too little paint means a second trip and possibly a slightly different colour batch; buying too much wastes money. Enter the size of the room, how many doors and windows it has, how many coats you plan and the coverage printed on the tin, and the calculator gives the litres you need, a 10% spare allowance and a sensible mix of tin sizes.
How to use the Paint Calculator
- Measure the room’s length, width and wall height in metres.
- Enter the number of doors and windows to leave out.
- Enter how many coats you will apply and the coverage from the tin (often 8 to 12 m² per litre).
- Choose whether to include the ceiling, then read the paint needed and suggested tins.
A 4 m × 3.5 m bedroom
The perimeter is 2 × (4 + 3.5) = 15 m, so the walls are 15 × 2.5 = 37.5 m². Taking away one door (1.9 m²) and one window (1.5 m²) leaves 34.1 m². Two coats at 10 m² per litre need 34.1 × 2 ÷ 10 = 6.8 litres, or about 7.5 litres with 10% spare: one 5 L tin and three 1 L tins. Adding the 14 m² ceiling would bring it to 9.6 litres.
Coverage, coats and surfaces
Coverage is how many square metres a litre of paint covers in one coat, and it is printed on the tin. Rough, porous or new plaster soaks up more paint, so expect less coverage and use a primer or a thinned first coat. Two coats are usual for walls; a big colour change, such as light over dark, may need three. Ceilings and walls are often painted in different finishes, so you may need separate tins. Doors and windows vary in size: the calculator uses a standard door of about 0.8 × 2 m and a window of about 1.2 × 1.25 m. Keep a little leftover paint, labelled with the room, for touch-ups.
Frequently asked questions
How much does a litre of paint cover?
Most interior wall paints cover about 8 to 12 m² per litre per coat on smooth, primed walls. Check the tin.
Do I need two coats?
Usually, yes. One coat rarely gives even colour, especially on new plaster or when changing colour.
Should I subtract doors and windows?
Yes for a fair estimate, but the 10% spare covers small differences and cutting in.
How much paint for a ceiling?
The ceiling area is length × width. A 4 × 3.5 m room has a 14 m² ceiling, about 2.8 litres for two coats at 10 m² per litre.