Paint calculator
The coverage printed on a tin is measured on a sealed, previously painted wall. Bare plaster, new filler and anything porous can take half again as much on the first coat. If you are painting fresh plaster, thin the first coat as a mist coat and treat the tin figure as optimistic — then the second and third coats land right on it.
Paint needed is wall area times the number of coats, divided by the coverage rate. Emulsion covers about 11 m² per litre on a sealed wall, so a 4 × 3 m room with 2.4 m walls takes roughly 5.6 litres for two coats after deducting a door and a window.
How to work out paint
Two things push real usage past the calculation. The first is porosity: new plaster, filler and old chalky paint all absorb far more than the tin promises, which is why a mist coat exists. The second is colour change. Going from a dark wall to a pale one is genuinely a three-coat job however good the paint claims to be, and a tinted primer will do it faster and cheaper than a third coat of expensive topcoat. Ceilings are the opposite problem — they look smaller than they are and are almost always underestimated.
Questions
A 12 × 12 ft room with 8 ft walls is about 350 ft² of wall after a door and window. That is roughly 2.5 litres per coat, so 5 litres for two coats.