Paint Calculator
Litres of paint for walls and ceiling, by coat.
How it works
Paint quantity is area times coats divided by the coverage rate on the tin. The rate is the number that varies most: 10 m² per litre is typical for emulsion on a prepared surface, but bare plaster, textured wallpaper or a dark colour underneath can cut it by half.
The first coat on a new surface always uses more than later ones, because the substrate absorbs it. Two coats is the realistic minimum for an even finish, and three if you are painting light over dark. Subtract windows and doors, but do not bother subtracting skirting — the extra is well inside the margin of error.
Related calculators
How the Paint Calculator works
A paint calculator works out the volume needed from wall and ceiling area, the number of coats and the coverage rate on the tin. Openings are subtracted and the ceiling can be included or left out.
Coverage is the variable that moves
The area is straightforward: perimeter times height, minus windows and doors. All the uncertainty sits in the coverage rate. For emulsion on a prepared surface, 10 m² per litre per coat is typical — but that figure halves easily.
Bare plaster and plasterboard absorb far more than a primed wall. Textured paper increases the actual surface area. A dark base under a light colour needs an extra coat. The number on the tin is an optimistic laboratory figure over an ideal substrate, and should be treated as such.
How many coats
Two coats is the realistic minimum for even colour. One coat almost always shows through, particularly where roller passes meet. Three are needed when covering dark with light, or with strongly pigmented colours that cover poorly on their own.
The first coat on a new surface always uses more than later ones, because the substrate absorbs it. Priming pays for itself: it costs less than paint and noticeably reduces how much of it you use.
FAQ
How much paint does a 20 m² room need?expand_more
At 2.7 m high, a 5 × 4 m room has 18 m of perimeter and almost 49 m² of wall, or about 44 m² after openings. Two coats at 10 m² per litre is roughly 9 litres. Add about 4 more if you are painting the ceiling too.
Should I subtract windows and doors?expand_more
Yes — in an ordinary room that is 4–6 m², more than a litre of paint. Subtracting skirting and architrave is not worth the effort: the difference sits well inside the margin of error on coverage.
What should I do with leftover paint?expand_more
Keep about a litre for touch-ups in a tightly sealed tin. Write the colour name and tint code on it: matching a shade again from a sample is nearly impossible, but any branch will reproduce it exactly from the code.