How much paint do you actually need?

7 minute read · updated 2026-10-19

Quick answer: Measure the wall area (room perimeter × height), take off about 1.5 m² for each door and window, multiply by the number of coats (usually two), divide by the coverage on the tin (typically 10–14 m² per litre for emulsion), then round up to whole tins.

Buying paint is one of those jobs where guessing costs money in both directions. Too little and you are back at the shop mid-job, hoping the new tin matches. Too much and you have half a tin going skinny in the shed. The maths is simple, and it takes two minutes.

Start with the wall area

You need the area of the surface, in square metres. For a whole room, measure the perimeter and multiply it by the height. Walk round the room with a tape, add the length of each wall together, then multiply by floor-to-ceiling height. That one sum covers all four walls at once and is quicker than measuring them separately.

For a single wall, length times height.

Doors and windows

Doors and windows are holes you are not painting, so subtract them. A standard internal door is about 1.5 square metres. A medium window is about 1.5 square metres too, though they vary a lot more.

Whether it is worth the effort depends on scale. In a small bedroom with one door and one window, you are taking roughly 3 square metres off perhaps 35, which is under a tenth. Leaving them in simply gives you a little spare paint, which is no bad thing. In a room with patio doors, a bay window and two internal doors, the openings can be a quarter of the wall area and you should definitely subtract them. The rule of thumb: ignore openings under about 2 square metres each, subtract anything bigger.

What the coverage figure on the tin means

Every tin prints a coverage figure, usually somewhere between 10 and 14 square metres per litre for emulsion. That number is for one coat, on a sound, prepared, previously painted surface, applied by someone not being generous. It is a best case.

Real coverage drops when the surface is textured, absorbent or bare, when you are rolling rather than brushing, and when the paint is a thin one-coat trade emulsion being pushed over a dark colour. Use the tin's figure, then expect to be a bit short of it rather than ahead.

How many coats

Two coats is normal and should be your default. One coat almost never looks even, particularly in daylight across a long wall.

Plan for three coats when you are going light over dark, painting a strong colour such as a deep red or yellow where the pigment covers poorly, or covering patch repairs and filler that will drink the first coat unevenly.

Fresh plaster is a separate case. New plaster is porous and will suck the moisture out of full-strength paint, which then sits badly and can peel. Give it a mist coat first: emulsion thinned with water, usually around three parts paint to one part water, brushed or rolled on as a sealing layer. Let the plaster dry out properly first, which can take weeks on a new wall, then mist coat, then two full coats.

Ceilings, woodwork and primer

A ceiling is just length times width. It is the same arithmetic as the floor, and it often needs two coats because ceilings show every lap mark.

Woodwork is measured differently and does not scale from wall figures at all. Skirting, architrave, doors and window frames are small areas, but gloss and satinwood go further per litre and you need separate tins of primer, undercoat and topcoat. A 750 ml tin of topcoat does a lot of skirting.

Primer is for bare wood, bare metal, filler and stains. On sound painted surfaces you do not need it.

A worked example

Take a bedroom 4 m by 3.5 m, with a ceiling height of 2.4 m. One door and two windows.

Perimeter: 4 + 3.5 + 4 + 3.5 = 15 m.
Wall area: 15 x 2.4 = 36 square metres.
Openings: one door at 1.5, two windows at 1.4 each, so 4.3 square metres. Take that off: 36 - 4.3 = 31.7 square metres.
Two coats: 31.7 x 2 = 63.4 square metres of painting.
Coverage on the tin says 12 square metres per litre: 63.4 / 12 = 5.3 litres.

So you need 5.3 litres of emulsion for the walls. Tins come in 2.5 litre and 5 litre sizes, so buy a 5 litre tin and a 2.5 litre tin. That leaves spare for touch-ups, which you will want.

The ceiling: 4 x 3.5 = 14 square metres, two coats is 28, divided by 12 is 2.4 litres. One 2.5 litre tin of ceiling paint is cutting it fine, so take 5 litres if the ceiling is patchy or you are covering a colour.

Rounding up

Always round up to whole tins, and always buy the paint for one room in one go. Tinted colours are mixed in batches and two tins bought weeks apart can differ just enough to see where you stopped. If you are mixing from more than one tin for a big wall, pour them together into a bucket first so the colour is identical across the whole surface.

If you would rather have the sums done for you, Tantoo has a paint calculator that takes room dimensions, openings and number of coats and tells you the litres and the tin sizes, alongside calculators for wallpaper, flooring, tiles and plasterboard. It is free and needs no account. You can also save a room and come back to it when the next job comes round.