Work out litres and tins
Room size, what you are painting, coats and the spreading rate from the tin. Mist coat for new plaster is optional.
Mist coat counted as one extra coat of paint thinned 3 parts paint to 1 part water. Tins are picked to waste the least paint, then to use the fewest tins.
Coverage is approximate and changes with the surface, colour and application. Check the tin for the spreading rate.
How much paint for a room?
Quick figures for common room sizes.
| Room | Wall m² | Walls | Buy | Ceiling |
|---|---|---|---|---|
| 3 × 3 m | 25.7 | 4.3 L | 1 × 5 L | 1.5 L |
| 4 × 3 m | 30.5 | 5.1 L | 1 × 5 L + 1 × 2.5 L | 2.0 L |
| 4 × 4 m | 35.3 | 5.9 L | 1 × 5 L + 1 × 2.5 L | 2.7 L |
| 5 × 4 m | 40.1 | 6.7 L | 1 × 5 L + 1 × 2.5 L | 3.3 L |
| 6 × 5 m | 49.7 | 8.3 L | 1 × 10 L | 5.0 L |
Door 1.6 m² and window 1.5 m² taken off. Add a mist coat on new plaster.
How to work out paint
Wall area = perimeter × height, less doors and windows. Ceiling area = length × width. Litres = area × coats ÷ spreading rate.
Spreading rate
Every tin gives a spreading rate in m² per litre per coat. Retail matt emulsions typically quote up to about 13–14 m²/L, and some trade contract matts up to around 16–18 m²/L on smooth, sealed surfaces. On real walls you rarely get the “up to” figure, so the calculator starts at 12 m²/L — change it to suit the product and the surface. Rough, porous or textured surfaces and strong colour changes all use more.
Mist coat on new plaster
Bare plaster soaks up paint. The usual first coat is a contract matt emulsion thinned with water — for example Dulux Trade says up to 1 part water to 3 parts paint for the first coat on new or bare surfaces. Leave the plaster to dry out fully first, and avoid vinyl paints for the mist coat because they can sit on the surface and peel.
Example
A 4 × 3 m room, 2.4 m high, with one door and one window: walls 14 × 2.4 − 3.1 = 30.5 m²; 2 coats at 12 m²/L = 5.1 L → buy 5 L + 2.5 L. Ceiling 12 m², 2 coats = 2 L → one 2.5 L tin.
Questions decorators ask
How much paint do I need for a room?
Work out the wall area (perimeter × height minus doors and windows), multiply by the number of coats and divide by the spreading rate on the tin. A 4 × 3 m room with 2.4 m walls needs about 5 litres of emulsion for two coats on the walls and about 2 litres for the ceiling.
How many m² does a litre of emulsion cover?
Most matt emulsions quote up to 12–14 m² per litre per coat, and some trade matts more. Real coverage is often lower on porous or rough surfaces, so allow for it.
Do I need a mist coat on new plaster?
Yes, bare plaster is very absorbent. Use a contract or trade matt emulsion thinned with water — Dulux Trade suggests up to 1 part water to 3 parts paint — before the normal coats. Make sure the plaster is fully dry.
How many coats of paint do walls need?
Usually two finishing coats, plus a mist coat on new plaster. A big colour change, especially dark to light, may need a third.
More free calculators
Tile calculator — Tiles, boxes, adhesive and grout› Flooring calculator — Laminate, LVT and engineered packs, underlay, skirting, screed› Plastering calculator — Bags of Multi-Finish, Hardwall or Bonding Coat› Plasterboard calculator — Boards, screws, tape and jointing for walls and ceilings› All free calculators for UK trades›Quote the job, not just the materials
Take-off, bill of quantities with labour rates and a quote the client signs by link — for decorators, tilers and floor fitters.
See Graftum estimatingCoverage is approximate and changes with the surface, colour and application. Check the tin for the spreading rate.