How Much Paint Do I Need?
The number on the tin is a laboratory figure for one coat on a smooth, sealed, similarly coloured surface. Almost nothing in a real house is any of those things, which is why people run out.
In short
- Wall area is the room perimeter multiplied by the height.
- Only deduct openings larger than about 1 m²; small ones are absorbed by waste.
- Two coats is the normal case, not the exception.
- The first coat on bare or porous surfaces can use 30–50% more than the stated coverage.
Measuring the walls
Add up the lengths of all the walls to get the perimeter, then multiply by the floor-to-ceiling height. For a room 5 m by 4 m and 2.5 m high, the perimeter is 18 m and the wall area is 18 × 2.5 = 45 m².
The ceiling, if you are painting it, is simply length × width — 20 m² for the same room, giving 65 m² in total. Note that a room twice the floor area does not have twice the wall area: doubling the floor space of a square room multiplies the perimeter by about 1.41, not by 2.
Which openings to deduct
The usual advice is to ignore doors and windows entirely, which is sound for one or two ordinary openings — the paint you save is roughly the paint you waste on the roller tray, the edges and the touching-up.
For anything substantial it is worth deducting. A standard door is about 1.6 m², a typical window between 1 and 2 m². Once a room has patio doors or a large picture window, ignoring them can overstate the requirement by 10% or more.
A workable rule: deduct openings larger than about 1 m², ignore anything smaller, and never deduct more than about half the total wall area no matter how glazed the room is, because the remaining walls still need cutting in around every one of those openings.
| Room 5 × 4 m, 2.5 m high | Area |
|---|---|
| Walls (18 m perimeter × 2.5 m) | 45.0 m² |
| Ceiling (5 × 4) | 20.0 m² |
| Less one door (1.6 m²) and one window (1.7 m²) | −3.3 m² |
| Total to paint | 61.7 m² |
| At 12 m²/L, two coats | 10.3 litres |
Why the tin overstates what you will get
Stated coverage is typically 10–14 m² per litre for emulsion, measured under conditions chosen for consistency: a smooth, sealed, non-absorbent surface, applied at the intended film thickness, in one coat.
Real surfaces take more. Bare plaster is porous and drinks the first coat — a mist coat thinned with water is standard practice precisely because the plaster will otherwise absorb an expensive full-strength coat. Textured or previously matt-painted walls have more surface area than their measured dimensions suggest. Application method matters too: rollers waste more than brushes, and spraying wastes more than either.
A practical allowance is to expect 30–50% more consumption than the stated figure for a first coat on bare or porous material, and close to the stated figure for subsequent coats on a sealed surface.
How many coats you actually need
Two is the default for any colour change. One coat is enough only when refreshing the same colour on a sound, sealed surface.
Three becomes likely in two situations. The first is a strong colour change in either direction — covering a dark wall with a pale one, or building up a saturated colour whose pigment gives poor opacity. Strong reds and yellows are notorious for this, because the pigments that make them vivid are not the ones that make them opaque.
The second is bare plaster or new filler, where the mist coat is doing sealing work rather than colouring, so you need it plus two proper coats.
| Situation | Coats to allow |
|---|---|
| Same colour, sound sealed wall | 1 |
| Normal colour change | 2 |
| Light over dark, or a strong red or yellow | 3 |
| New plaster | Mist coat + 2 |
Buy in the right tin sizes
Paint is sold in fixed sizes, so the real question is which combination of tins covers your requirement with the least waste. Needing 10.3 litres means either three 5 L tins with 4.7 L left over, or two 5 L plus a 2.5 L, which leaves 2.2 L.
Buying slightly over is the right error to make. Running out mid-wall means a second trip, and — more seriously — a different batch. Manufacturers mix colour in batches and there can be a visible difference between them, which is why the batch number is printed on the tin. If you are buying more than one tin of the same colour, buy them together and check the batch numbers match, or mix them together in a larger container before starting so any variation is spread evenly.
The order that saves paint
Ceiling first, then walls, then woodwork. Working top down means splashes land on surfaces you have not finished yet, and doing woodwork last avoids masking it twice.
Cut in the edges of each coat immediately before rollering that coat, not all the edges first. Cut-in paint that has dried has a different texture from rollered paint, and the difference is visible in raking light — doing them wet-on-wet blends the boundary.
Frequently Asked Questions
How do I calculate the wall area of a room?
Add the lengths of all the walls to get the perimeter, then multiply by the floor-to-ceiling height. A room 5 m by 4 m with a 2.5 m ceiling has an 18 m perimeter and 45 m² of wall. Add length × width for the ceiling if you are painting it.
Should I subtract doors and windows?
Deduct openings larger than about 1 m² and ignore smaller ones, since the paint saved is roughly what you waste on the tray and touching up. A standard door is about 1.6 m² and a typical window 1 to 2 m². Do not deduct more than about half the wall area regardless.
How many coats of paint do I need?
Two for any colour change. One is enough only when refreshing the same colour on a sound sealed surface. Allow three for light over dark or for strong reds and yellows, whose pigments cover poorly, and a mist coat plus two on new plaster.
Why did I use more paint than the tin said?
Coverage figures assume a smooth, sealed, non-absorbent surface at the intended film thickness. Bare plaster absorbs far more, textured walls have more real surface area than their dimensions suggest, and rollers and sprayers waste more than brushes. Allow 30–50% extra for a first coat on porous material.
Does it matter if I buy tins from different batches?
It can. Colour is mixed in batches and there may be a visible difference between them, which is why the batch number is on the tin. Buy tins of the same colour together and check the numbers match, or decant them into one container and mix before starting.