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Paint Calculator
Litres or gallons by wall area, coats and surface.
What it does. Paint quantity is wall area divided by the coverage rate, multiplied by the number of coats. The coverage printed on the tin assumes a smooth, sealed, similarly coloured surface — bare plaster, textured walls and strong colour changes all use noticeably more.
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How to use Paint Calculator
- Enter room dimensions, or the wall area if you already have it.
- Set coats, surface type and how many doors and windows to deduct.
- Read the quantity, and buy to the next tin size.
Why the tin over-promises
Coverage figures on a tin are measured under ideal conditions: a smooth, sealed, primed surface in good light with an even roller. Real walls are rougher than that.
Bare plaster and new plasterboard absorb the first coat substantially — a mist coat on new plaster can use half again what the tin suggests.
Texture is the other multiplier. A heavily textured or artexed wall has considerably more actual surface than its measured area, and the paint goes where the surface is, not where the tape measure went.
How many coats you actually need
| Situation | Coats |
|---|---|
| Same colour, refresh | 1–2 |
| Light over light | 2 |
| Dark over light | 2–3 |
| Light over dark | 3–4, or prime first |
| Bare plaster | Mist coat + 2 |
| Strong colour (red, yellow) | 3+, pigment dependent |
Light over dark is where people underestimate badly. Rather than three or four finish coats, a tinted primer is cheaper and gives a better result — it is designed to block, and finish paint is not.
Deductions, and when not to bother
A standard door is roughly 1.6 to 2 square metres and a typical window 1 to 1.5. Deducting two doors and two windows from an average room removes about 6 square metres — real, but under one litre at two coats.
For one room the deduction rarely changes what you buy, because tins come in fixed sizes.
It matters when you are painting a whole house, or when the deduction happens to sit right on a tin boundary. Otherwise, rounding up beats precision — and having to match a second batch of tinted colour is worse than a spare half litre.
Frequently asked questions
How much paint do I need for a room?
Wall area divided by the coverage rate, times the number of coats. For a typical 4×3m room with 2.4m ceilings that is about 33 square metres of wall, or roughly 5 litres for two coats at normal coverage.
Why does my paint not go as far as the tin says?
Tin coverage assumes a smooth, sealed, primed surface. Bare plaster absorbs heavily, and textured walls have more real surface than their measured area — the paint goes where the surface is, not where the tape measure went.
How many coats do I need to cover a dark colour?
Three to four coats of ordinary finish paint, or one tinted primer plus two finish coats. The primer route is cheaper and gives a better result, because blocking is what primer is designed to do.
Should I subtract doors and windows?
For one room it rarely changes what you buy, since tins come in fixed sizes. It matters across a whole house, or when the deduction sits right on a tin boundary.
Is it better to buy too much or too little?
Too much. Running short means matching a second batch of tinted colour, and mixed batches can differ slightly — visible on a large wall in daylight.