The short answer, by paint type
Coverage is quoted in square metres per litre, per coat. The figures below are the practical middle of the range you will see across UK brands, on a smooth surface that has been painted before. They are the numbers to plan with when you have nothing better to go on.
| Paint | Per litre, one coat | A 2.5 L tin | A 5 L tin |
|---|---|---|---|
| Matt emulsion | 12 to 14 m2 | 30 to 35 m2 | 60 to 70 m2 |
| Silk or soft sheen emulsion | 12 to 15 m2 | 30 to 37 m2 | 60 to 75 m2 |
| Gloss, satin, eggshell | 14 to 16 m2 | 35 to 40 m2 | 70 to 80 m2 |
| Masonry paint | 5 to 10 m2 | 12 to 25 m2 | 25 to 50 m2 |
| Primer or undercoat | 10 to 12 m2 | 25 to 30 m2 | 50 to 60 m2 |
| Wood stain or fence treatment | 6 to 10 m2 | 15 to 25 m2 | 30 to 50 m2 |
The spread on masonry paint and fence treatment is not sloppiness. A smooth render and a rough pebbledash are genuinely different surfaces, and rough ones can take twice the paint for the same visible area. If you are painting anything textured, plan from the bottom of the range.
Turning a room into litres
Coverage only helps once you have an area. For walls, add up the length of every wall to get the perimeter, then multiply by the floor-to-ceiling height. A room 4 m by 3 m with a 2.4 m ceiling has a perimeter of 14 m, so the wall area is 14 x 2.4 = 33.6 square metres.
Then divide by coverage and multiply by the number of coats. At 12 square metres per litre, those walls need 33.6 / 12 = 2.8 litres for one coat, so 5.6 litres for two. In tin sizes that is a 5 litre plus a 2.5 litre, because paint is not sold by the exact litre. The paint calculator does this conversion for you, including the coats and the ceiling, if you would rather not do it by hand.
Ceilings are measured separately: floor length times width, so 4 x 3 = 12 square metres in the same room. Ceilings usually take two coats too, and ceiling paint is often a slightly different formulation from the wall emulsion.
Why bare plaster changes the sum
New plaster is porous. The first coat is partly absorbed rather than sitting on the surface, so it covers far less than the tin promises, and if you put full-strength emulsion straight onto it, the paint can dry unevenly or peel later. The standard fix is a mist coat: emulsion thinned with roughly 10 to 20 percent water, applied first to seal the surface.
Budget for that mist coat as an extra coat. A newly plastered room is realistically three coats: mist, then two finish coats. That is why the same room can need 5 litres in one house and 10 in another.
The other things that eat paint
- Colour changes. Light over dark needs an extra coat, sometimes two. Strong reds and yellows have poorer opacity than most colours and often need three regardless of what is underneath.
- Texture. Woodchip, artex, brick and rough render all have more real surface area than their measured area. Assume the low end of the coverage range.
- Application. A roller with a long pile holds and lays off more paint than a short pile. Spraying uses more paint than rolling for the same finish, because of overspray.
- Patch repairs. Filler and freshly skimmed patches are porous in exactly the way new plaster is, and will flash through the finish coat unless spot-primed first.
A worked example
Take a 3.5 m by 3 m bedroom with a 2.4 m ceiling, previously painted magnolia, going to a mid-grey matt emulsion. One door, one window.
- Perimeter: (3.5 + 3) x 2 = 13 m
- Wall area: 13 x 2.4 = 31.2 m2
- Openings: one door and one standard window, roughly 3.5 m2 together. Call the walls 28 m2.
- Two coats at 12 m2 per litre: 28 x 2 / 12 = 4.7 litres
- Buy 5 litres, and expect a little left for touch-ups.
Add the ceiling separately: 3.5 x 3 = 10.5 m2, two coats at 12 m2 per litre is 1.75 litres, so a 2.5 litre tin of ceiling paint.
Rules of thumb worth remembering
- 12 square metres per litre per coat is the emulsion default.
- Always plan two coats. One coat is a repaint in the same colour, not a finish.
- Bare plaster means three coats, the first one thinned.
- Round up to the next tin. Running out mid-wall leaves a visible line.
- Keep the leftover, labelled with the room. Touch-ups from a different batch rarely match.
If you want the arithmetic done for you, put your room dimensions into the paint calculator. For a larger project, the tile calculator and gravel calculator work the same way, and the rest of the Calc Garden guides cover the other jobs.