My tin says it covers 30 m². What do I type?

Divide by the tin size. 30 m² from a 2.5 L tin is 12 m² per litre. BuildBatch asks for the per-litre figure and refuses one over 25, because that usually means the whole-tin figure was typed.

Paint coverage explained: m² per litre, per coat

Coverage is how far one litre of paint goes in one coat, written as square metres per litre. It changes a paint estimate more than any other figure, and it comes from the tin, not from a calculator.

Per litre, and per coat

Tins state coverage in different ways:

“Up to 12 m² per litre” is already the figure you need.
“Covers up to 30 m²” on a 2.5 L tin means 30 ÷ 2.5 = 12 m² per litre.
A range, such as 10 to 12 m² per litre, means the surface decides. Use the lower figure for an estimate.

The estimate needs the figure per litre and per coat. Two coats means the area is covered twice, so the litres double.

Why the example is 10 m² per litre

BuildBatch starts at 10 m² per litre so that you can get an answer before you have a tin. It is on the cautious side of what most emulsions state, so an estimate left on the example errs towards slightly too much paint. The field carries an “Example value” tag until you change it, and the copied list says “example value, check your tin”.

The form also checks the figure. Under 5 or over 17 m² per litre gets a warning: unusual, but possible. Over 25 is refused, because it almost always means the whole tin’s figure was typed, and that mistake understates the paint needed, which is the direction that leaves you short on a Sunday afternoon.

Worked example

Net wall area 34.484 m², two coats, 10% extra.

Coverage	Before the extra	With 10% extra
10 m² per litre	6.897 L	7.586 L
12 m² per litre	5.747 L	6.322 L
14 m² per litre	4.926 L	5.419 L

Each figure is worked from the unrounded one before it, as the calculator does, so multiplying the rounded figures can differ in the last decimal place.

Going from 10 to 14 m² per litre saves 2.17 L. The whole 10% allowance is 0.69 L. Getting the coverage right matters about three times more than fine-tuning the allowance.

What makes real coverage lower than the tin says

Bare or new plaster soaks up paint. It needs a mist coat or a primer-sealer first, and the top coats go further afterwards. The paint’s instructions decide.
Textured, rough or patched surfaces have more surface than their flat area, and filled patches are more absorbent than the wall around them.
A strong colour change may need an extra coat rather than more paint per coat.

The tin’s figure is for its stated conditions, usually a sound, sealed, smooth surface. Anything else means check first.

What to type

Find the coverage on the tin or on the maker’s data sheet.
If it is given per tin, divide by the litres in the tin.
If it is a range, use the lower figure.
If it is not clear whether the figure is per coat, treat it as per coat and keep the coats at 2. If that reading is wrong you will have slightly too much, not too little.
Type it in. The litres and the tins update as you type.

Do it with BuildBatch

The paint calculator keeps the coverage field one tap away under “Change paint details”, and its results offer “Want a closer estimate?”, which opens the field for you. Change the figure and the tins update.

What this guide does not cover

Which paint to choose, sheen and finish, primers and undercoats, spraying, and exterior or masonry paint, which has its own coverage figures and its own rules. The tin, and the maker’s data sheet, are the source for all of those.