Paint Calculator

How this is worked out

Wall area is the perimeter multiplied by the height, less the doors and windows:

area = 2(length + width) × height − openings

Then the paint is that area multiplied by the number of coats, divided by the coverage rate, plus a waste allowance:

litres = (area × coats ÷ coverage) × (1 + waste)

Coats multiply the area to cover, not the coverage rate — two coats on 30 m² is 60 m² of painting. It is an easy thing to get backwards and it halves or doubles the answer.

Coverage differs by paint. Emulsion covers around 12 m² per litre; gloss around 16 because it goes on thinner; primer on bare plaster only about 10, because a porous surface drinks it. The figures here are typical and your tin will state its own — use that if it differs.

The tin plan rounds up, on purpose. Running out mid-wall leaves a visible edge where the new tin starts, and paint mixed in a different batch can differ slightly in shade. Buying the next size up is usually cheaper per litre than buying two smaller tins as well.

A worked example

Doors and windows are the part people skip

A standard door is about 1.98 m² and a standard window about 1.44 m². In a small room with two doors and a window that is over five square metres — around fifteen per cent of the wall area, and close to a litre of paint across two coats.

Leaving them in does not cause disaster; it causes over-buying, which is money spent on a tin that sits in a shed until it goes off. Emulsion keeps a couple of years unopened and much less once opened, so the leftover is rarely as useful as it feels at the time.

The exception is if you are painting the door and frame in the same colour, in which case leave the door count at zero for the wall calculation and use the gloss coverage figure for the trim separately — they are different paints with different coverage.

The second coat is not optional, and the first one is not the finish

One coat almost never covers properly, especially over a colour change or onto fresh plaster. What you get is patchy density that looks acceptable while wet and reveals itself as the paint dries — usually as roller lines and a visible difference where the previous colour shows through.

Going from dark to light can need three coats, or two coats over a tinted primer, which is cheaper and faster than three coats of expensive topcoat. Going from light to dark is often two coats of topcoat with no primer, because the covering direction is easier.

Bare plaster needs a mist coat first — emulsion thinned with water, roughly 70:30 — which soaks in and seals the surface. Putting undiluted paint straight onto new plaster is how it peels off in sheets a year later.

Why the waste allowance exists

Ten per cent is not padding for the shop's benefit. A meaningful amount of paint never reaches the wall: it stays in the tray, soaks into the roller sleeve, and dries on the brush. Add cutting in, touch-ups after the tape comes off, and the inevitable spill, and the loss is real.

For a textured surface — heavy woodchip, pebbledash, rough render — raise it substantially. A textured wall has far more actual surface area than its measured dimensions suggest, and coverage can drop by a third or more.

Keep whatever is left over, sealed and labelled with the room name. Touch-up paint from the same tin matches; a new tin of the same colour from a different batch often does not, and the patch will be visible in daylight.

Buying the right tins

Paint comes in fixed sizes — commonly 10 L, 5 L, 2.5 L, 1 L and 0.75 L. The calculator translates a litre figure into a shopping list rather than leaving you to work out at the till whether 5.53 litres means one big tin or three small ones.

Larger tins are almost always cheaper per litre, so where the choice is between a 5 L tin and two 2.5 L tins, the single larger tin usually wins on both price and colour consistency.

One genuine caution on colour: if you are having a colour mixed rather than buying a stock shade, buy all of it at once. Mixed colours are made to a formula but small variations between batches do happen, and a second tin mixed weeks later on a different machine is the classic cause of a wall that looks subtly wrong in one corner.

Assumptions and sources

Coverage rates

Typical manufacturer figures: emulsion ~12 m²/L, gloss ~16, primer ~10, masonry ~8, ceiling paint ~13. Your tin states its own — use that where it differs.

Standard openings

Door taken as 1.98 m² (2.0 × 0.99 m) and window as 1.44 m² (1.2 × 1.2 m). Adjust the counts if yours differ materially.

Waste allowance

10% by default, covering tray and roller retention, cutting in, touch-ups and spillage. Raise it for textured surfaces.

Tin sizes

Common retail sizes of 10, 5, 2.5, 1 and 0.75 litres. The plan rounds up deliberately.