Paint calculator

Paint is bought by the litre and spread over wall area, so the room’s floor size is the wrong number to start from — what matters is the perimeter, the ceiling height, and how much of the wall is taken up by doors and windows. Enter the room below and you will get the litres for the walls.

How the figure is worked out

Wall area is the perimeter multiplied by the ceiling height. A 12 ft by 12 ft room has a 48 ft (14.6 m) perimeter, so at a 2.4 m ceiling that is about 35 m² of wall. Take off roughly 3.4 m² for a standard door and one window and you are painting about 32 m². At 12 m² per litre per coat, two coats needs about six litres.

Twelve square metres per litre is a typical figure for emulsion on a wall that has been painted before. Treat it as a starting point rather than a promise — the tin will state its own coverage, and that is the number to trust.

When you will need more

  • Bare plaster drinks the first coat. Add a mist coat — watered-down emulsion — and count it as an extra coat.
  • Going lighter over a dark colour often takes three coats, sometimes four for strong reds and blues.
  • Textured walls have more surface area than their dimensions suggest.
  • Rollers and trays hold onto a surprising amount of paint that never reaches the wall.

The ceiling is separate

The calculator covers walls only. For the ceiling, use the floor area — they are the same size — and divide by the same coverage figure. A 144 sq ft room is 13.4 m², so two coats at 12 m² per litre is about three litres. The square footage calculator gives you that area.

Buy in sensible tin sizes

Paint comes in 2.5 L and 5 L tins far more often than in single litres, so round up to what is actually on the shelf. If two rooms are being done in the same colour, work out both and buy one larger tin — it is usually cheaper per litre and guarantees a consistent batch.

Common questions

How much paint do I need for a 12x12 room?
About six litres for two coats on the walls, assuming a 2.4 m ceiling and one door and window deducted. Add roughly three more litres if you are painting the ceiling too.
How many coats of paint does a wall need?
Two is standard on a previously painted wall in a similar colour. Allow three when going lighter over a dark colour, and add a mist coat on bare plaster before the two topcoats.
Should I subtract doors and windows?
Yes, if you want an accurate figure. A standard 826 mm door is about 1.7 m² and an average window about the same, which is why 3.4 m² is the default above. In a room with a lot of glazing the difference is worth a whole tin.
How much does a litre of paint cover?
Around 12 m² per coat for emulsion on a prepared wall, though it ranges from about 6 to 16 m² depending on the paint and the surface. Always check the tin.

Also useful: flooring calculator and square footage calculator.