Paint calculator

How many litres — and which combination of tin sizes — to buy for a room, with the coverage rate adjusted for what the surface actually drinks on the first coat.

Room

Length

Width

Wall height

Paint the ceiling too Paint the walls

Extra wall area For an L-shaped room, a stairwell, or a second room on the same tin.

m²

Openings to deduct

Doors

Windows

Area per door

m²

Area per window

m²

Ignore openings smaller than 1 m² Painters normally do not deduct small openings — the cutting-in around them uses the paint you saved.

Surface and coats

Surface condition

Coats

Coverage Override with your tin's figure if you have it.

m²/L

Include primer or a mist coat Also paint doors, frames and skirting in trim paint

Buying

Tin sizes available

Currency

Price per L

Primer per L

Paint to buy

—

Gross wall area—
Openings deducted—
Net wall area—
Ceiling area—
Total area per coat—

Paint needed

First coat—
Later coats—
Primer—
Trim paint—

Coverage is the rate a roller actually achieves, not the tin's best-case figure. Deep colours over a light base often need a third coat whatever the tin says.

How this calculator works

The first coat is not the same as the second

This is the part almost every paint calculator gets wrong. A tin that claims 12 m² per litre will do roughly that on a surface that has been painted before. On bare plaster, new plasterboard or sanded filler, the substrate absorbs paint and the first coat covers perhaps 75% of that. Multiply by two coats and the error is a whole tin on an average room.

So the calculator prices coat one and the later coats separately. Pick the surface condition and it applies a first-coat factor from the coverage rate table: 0.75 for new plaster and plasterboard, 0.70 for render and masonry, 1.00 for a wall that has already been painted. Tick the primer box and the first-coat penalty disappears — that is what the primer is for.

Openings under a square metre are not deducted

By default the calculator ignores any opening smaller than 1 m², which is how a decorator estimates. The reason is not laziness: cutting in carefully around a small window frame with a brush uses more paint per square metre than rolling the open wall, and the two effects roughly cancel. Deducting every small opening systematically leaves you short.

It works out the tin combination, not just the litres

Knowing you need 7.3 litres is only half the answer, because nobody sells 7.3 litres. Two 5-litre tins wastes 2.7 litres; one 5-litre plus one 2.5-litre wastes 0.2. The calculator searches the standard tin range — 10, 5, 2.5 and 1 litre, or 5, 1 and ½ US gallons in imperial — for the cheapest combination that covers the job, and tells you what to put in the trolley.

Buy the wall paint in one batch and box it. If the job needs more than one tin of the same colour, pour them all into a single larger bucket and stir before you start — decorators call this boxing. Factory batches of the same colour code vary slightly, and a batch change halfway up a wall shows in daylight.

Coverage rates used

Practical coverage per litre, per coat. Full table and reasoning on the coverage rates page.
Surface	m²/L	ft²/gal	First coat

Common questions

Why does my tin claim better coverage than this?

Tin figures are measured under laboratory conditions on a sealed, smooth, primed panel at a controlled film thickness. A roller on a real wall lays down a thicker and less even film, and some paint stays in the tray and the roller sleeve. Treat the tin figure as a ceiling, not an estimate. If you want to use it anyway, type it into the coverage box and the calculator will use your number.

How much paint does a deep colour need?

Plan for three coats whenever you are going dark over light, or using a saturated red, orange or yellow — those pigments have poor opacity and are the classic four-coat disasters. A tinted grey primer under a deep colour will usually save you a full finish coat, and primer is cheaper than the finish.

Do I need to paint the ceiling and walls with the same product?

No, and normally you would not. Ceilings take a flat matt, walls take a matt or eggshell in a scrubbable grade, trim takes a satin or gloss. The calculator gives you the ceiling area as its own line so you can price it separately; if you use one product for both, just add the two figures.

What about the mist coat on new plaster?

Fresh plaster must be sealed before a finish coat or the paint will peel. The traditional method is a mist coat — the same emulsion thinned roughly 30% with water — and it counts as your primer here. Tick the primer box; the calculator allows for it at about 11 m² per litre and removes the first-coat absorption penalty.

How much do I need for doors and skirting?

Tick the trim option. It allows both faces of each door, the frame and architrave, and a run of skirting around the room's perimeter, at the coverage rate for joinery. Trim paint is bought in small tins and one litre goes a long way — the usual mistake is buying far too much.

Paint coverage rate reference
Drywall calculator — for the boarding and jointing that comes before painting

Method last reviewed 2026-09-18. Estimates for planning; check your paint's technical data sheet before buying.