The formula
litres = area × coats ÷ coverage
Where it comes from
The wall area is the perimeter times the ceiling height. Off that come the openings: a standard door takes 1.69 m² and an ordinary window 1.2 m². The coverage figure is the manufacturer's, printed on the tin, and usually sits between 8 and 12 m² per litre per coat.
How to work it out by hand
- Add the four sides for the perimeter
- Multiply it by the ceiling height
- Subtract the area of doors and windows
- Multiply by the number of coats and divide by the coverage
What is worth knowing
Two coats are almost always necessary: one lets the old colour show through. Going from dark to light, count on three. The coverage on the tin is optimistic and measured on smooth, primed surfaces; on textured or porous walls it can halve. And buy it all on the same day from the same batch: shade varies between batches enough to see on the wall.