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Paint calculator
Measure the walls, subtract the openings, multiply by the coats. The number that trips people up is spread rate — it is on the tin and it varies more than you would think.
Check the tin. Manufacturers quote a best case on a sealed, smooth surface.
The calculation, and where it goes wrong
Wall area is the perimeter times the height, which for a rectangular room is 2 × (length + width) × height. Subtract the doors and windows, multiply by the number of coats, and divide by the spread rate.
Two things regularly throw the answer out. The first is forgetting that the second coat doubles the paint. The second is trusting the spread rate on the tin, which is measured under ideal conditions on a smooth, sealed, previously painted surface.
Spread rate is the variable that matters
| Surface | Realistic spread |
|---|---|
| Previously painted, smooth | 12–14 m²/L |
| Lightly textured | 10–12 m²/L |
| New plaster, unsealed | 8–10 m²/L |
| Bare masonry or render | 6–8 m²/L |
| Textured or artex ceiling | 6–9 m²/L |
New plaster in particular drinks the first coat. The standard approach is a mist coat — paint thinned with about 10–20% water — to seal the surface before the finish coats. Skipping it wastes far more paint than the mist coat uses, and the finish tends to peel.
How many coats you actually need
Two is the honest default. One coat only works when you are refreshing an identical colour on a sound surface, and even then the result often looks patchy in raking light.
Three is realistic when going dramatically lighter over a dark colour, covering a strong colour like a deep red, or painting bare substrate. A tinted primer does this job better than a third top coat: it is cheaper per litre and specifically formulated to block what is underneath.
Buy slightly over, once
Paint from different batches can differ subtly in tone, and the difference is most visible on a large flat wall in daylight. Buying everything at once, from the same batch, avoids the problem. It also leaves you a half-litre for touching up scuffs later, which is worth keeping — matching a colour two years on is far harder than storing a tin.
Trim, doors and skirting are usually calculated separately, since they use a different paint at a different spread rate. A rough rule is that trim in an average room takes under a litre, but the surface area of panelled doors is deceptive.
Common questions
How much paint do I need for a 12x12 room?
With 8 foot walls, one door and two windows, the walls come to roughly 32 m² after openings. Two coats at 12 m² per litre needs about 5.4 litres, so a 5 litre tin plus a small one. Adding the ceiling brings it to roughly 7.7 litres.
Do I subtract windows and doors?
Yes, when they are a meaningful share of the wall. A typical door is about 1.8 m² and a window about 1.4 m², so a room with a door and two windows loses roughly 4.6 m². In a room with a whole wall of glass, ignoring the openings could have you buying half again as much as you need.
Is one coat ever enough?
Only when repainting the identical colour over a sound, previously painted surface, and even then the result is often patchy where the roller overlapped. Any colour change, any bare substrate, and any sheen change needs two. Going light over dark usually needs a tinted primer plus two coats.
How much does a litre of paint cover?
Between 6 and 14 square metres depending on the surface and the paint. Tins quote the best case: smooth, sealed, previously painted walls. Textured surfaces and bare plaster are at the other end. Halving the quoted figure is a safer estimate for anything unsealed.