How this calculator works
The paintable area is the four walls (the perimeter × the ceiling height) minus the doors and windows. Multiply by the number of coats and divide by how many square metres a litre covers, usually about 12 for emulsion. The ceiling is length × width, worked out the same way.
Worked example: a 4 × 3 m room with a 2.4 m ceiling has 33.6 m² of wall. One door and one window take off 3.1 m², leaving 30.5 m². Two coats at 12 m² per litre is 30.5 × 2 ÷ 12 = 5.1 litres, so buy a 5 litre tin and a 1 litre tin, or one 5 L and one 2.5 L. The 12 m² ceiling needs another 2 litres.
Assumptions and sources
The assumptions behind your numbers (waste, sizes, rates) are listed in the notes directly under your results, and they update when you change an input.
The formula and a worked example are shown above, so you can check any result by hand. Read how we calculate and check our numbers. Results are estimates for planning: supplier products vary, so check the size printed on the label.
Tips before you buy
- New plaster needs a mist coat first: a watered-down coat of emulsion (about 70% paint to 30% water) that soaks in and seals it. Then apply your two full coats.
- Fill and sand cracks and holes, and wash greasy areas, before painting. Paint will not hide poor preparation.
- Use a contract matt (flat) emulsion on ceilings, which hides flaws, and a wipeable matt or silk on walls in kitchens and bathrooms.
- Buy all the paint for a room together and mix the tins into one container, so the colour is identical on every wall.
- Rough walls and dark colours cover less. Use the lower coverage figure, and consider a primer or a tinted undercoat for big colour changes.
- Houses built before about 1960 may have lead-based paint. Test before sanding or scraping, and do not dry-sand old paint.
Quick answers by size
Tools & materials for this project
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Frequently asked questions
How many litres of paint do I need for a 4m x 3m room?
About 5 litres for the walls with two coats, if the room is 2.4 m high with one door and one window, and another 2 litres for the ceiling.
How much does a litre of emulsion cover?
About 12 m² per coat on smooth, previously painted walls. The tin states its coverage, and bare plaster, rough walls and dark colours cover less.
What size tins does paint come in?
Most emulsion is sold in 1 litre, 2.5 litre, 5 litre and 10 litre tins, and larger tins are cheaper per litre.
Do I need two coats of paint?
Almost always. Two coats give an even finish and good coverage. Use a mist coat first on new plaster.
How much paint do I need for a 3 × 3 m bedroom?
About 4.3 litres for two coats on the walls, with a 2.4 m ceiling, one door and one window. See the 3 × 3 m room page for the ceiling and other coverages.
How much paint do I need for a ceiling?
The ceiling of a 4 × 3 m room takes 2.0 litres for two coats at 12 m² per litre. The how much paint for a room guide has a chart by room size.
How much does 2.5 litres of paint cover?
About 30 m² for one coat at 12 m² per litre, so about 15 m² of wall with two coats. The 2.5 litre tin page shows which rooms it does, and the tin coverage chart covers 1, 5 and 10 litre tins too.
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Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check the Building Regulations and your supplier's guidance before ordering.