How much wall will that paint cover?

A 2.5 litre tin of emulsion covers about 30 m² per coat. Two coats halve that, and nobody ever gets away with one.

The wall area Unit

Almost exactly 2.5 litre tin

30 m² is bigger than 2 of the 6 references on this page.

Reference figures are measurements and are rounded; the unit conversion behind them is exact.

Every reference on this page, with its size and its ratio to your number
Reference	Size	Yours ÷ theirs	Note
1 litre of emulsion	12 m²	2.5×	One coat on smooth plaster
average ceiling	12 m²	2.5×
2.5 litre tin	30 m²	1×
average room walls	40 m²	0.75×	A 4 × 3 m room, 2.4 m high
5 litre tin	60 m²	0.5×
10 litre tin	120 m²	0.25×

Emulsion covers about 12 m² per litre on smooth plaster but as little as 8 on bare or textured surfaces, which is where most underestimates come from.
Two coats is the norm; a strong colour change often needs three, or one coat of tinted primer plus two.

Frequently asked

Add the lengths of all the walls, multiply by the ceiling height, then subtract about 1.8 m² per door and 1.5 m² per window.

A typical 4 × 3 m room has about 40 m² of wall. At 12 m² per litre per coat, two coats need 6.7 litres — so a 5 litre tin plus a 2.5 litre tin.