How Much Paint Do You Actually Need?

Two things go wrong when buying paint. Buying too little means a second trip, and a risk that the new tin is a slightly different batch. Buying too much means leftover paint that will sit in a cupboard for six years and then go to a tip.

The arithmetic is not hard. The part people get wrong is the adjustments.

The base calculation

Wall area = perimeter × height.

For a room 4 m × 3 m with 2.4 m ceilings: perimeter is (4 + 3) × 2 = 14 m, so wall area is 14 × 2.4 = 33.6 m².

Subtract openings. A standard door is about 1.8 m², a typical window 1.5–2 m². With one door and one window, subtract roughly 3.5 m², leaving about 30 m².

Multiply by coats. Two coats is the realistic default, so 60 m² of coverage needed.

Divide by coverage. At a typical 12 m² per litre, that is 5 litres — so a 5 L tin, with the understanding that it will be close.

Why the tin's coverage figure is optimistic

Manufacturers quote coverage under favourable conditions: a smooth, sealed, previously painted surface, applied by roller at the intended thickness.

Real coverage is often 20–40% lower. The main causes:

Porous surfaces. New plaster, bare filler and old chalky emulsion absorb paint. New plaster in particular can take nearly twice as much on the first coat, which is why a thinned mist coat is standard practice — it seals the surface far more cheaply than full-strength paint does.

Texture. Woodchip, textured plaster or heavy brush marks increase actual surface area substantially. Bare brick or blockwork can halve coverage.

Colour changes. Painting light over dark, or over a strong colour, may need three coats. Reds, deep yellows and some oranges have weak opacity and often need three even over white — a known property of the pigments, not a defect.

Application method. Spraying loses paint to overspray. A roller with too long a nap holds more paint and lays it on thicker.

Typical real-world coverage, per litre:

Ceilings, and the thing people forget

Ceiling area is simply length × width — 12 m² for the room above. At two coats that is 24 m², or 2 litres.

Ceilings are usually painted with a dedicated matt emulsion, so this is a separate tin, not part of the wall figure. It is the most commonly forgotten item on a paint list.

Woodwork

Skirting, architrave, doors and window frames are estimated by length rather than area.

A rough rule: skirting and architrave consume about 1 litre per 60–70 m of run, per coat. A door needs roughly 0.25 litres per coat per side, including the frame.

Woodwork almost always needs primer or undercoat on bare wood, plus two topcoats. Budget three coats total.

Buying it

Round up to tin sizes, not to litres. Paint is sold in 1, 2.5, 5 and 10 L tins. If you need 6 litres, a 5 L plus a 2.5 L costs less than two 5 L tins and wastes less.

Check price per litre, not per tin. Larger tins are usually cheaper per litre, but not always, and promotions distort this constantly. The unit price comparator settles it in a few seconds.

Buy all of a colour at once, and check batch numbers match. Mixed-to-order colours can vary slightly between batches. If you must use two batches, decant them into a larger container and mix — "boxing" — so any variation is uniform rather than appearing as a line down a wall.

Keep about half a litre for touch-ups, in a small sealed container rather than a part-empty tin. A tin with more air than paint skins over within a year.

A worked example

A 4 × 3 m bedroom, 2.4 m ceilings, one door, one window, previously painted walls in a similar shade, plus ceiling and skirting:

The paint calculator handles rooms with several windows, non-rectangular layouts and different surface types, and shows the coverage assumption it used — which is the number to override if you are painting new plaster or going light over dark.