Paint Calculator
Work out how many litres (or gallons) of paint you need for a room, accounting for doors, windows, and the number of coats.
How this paint calculator works
The calculation is the one decorators use for estimating:
- Wall area = total wall length × wall height. "Total wall length" is the perimeter of the room — add up the length of every wall you're painting.
- Subtract openings. A standard door is about 1.6 m² (17 sq ft) and a standard window about 1.4 m² (15 sq ft). You don't paint those.
- Multiply by coats. Two coats is standard for a colour change or fresh plaster; one coat can be enough when repainting the same colour.
- Divide by coverage. Most emulsion/latex wall paint covers around 10–13 m² per litre (350–400 sq ft per US gallon). Check the tin of the paint you're buying — coverage varies by brand and surface.
We round the answer up, because running out of paint mid-wall is far worse than having half a litre spare for touch-ups later.
Tips for getting the estimate right
- New plaster drinks paint. Fresh plaster or bare drywall needs a mist coat and absorbs more — budget 20–30% extra, or reduce the coverage figure to about 9 m²/L.
- Dark to light needs more coats. Covering a deep colour with a pale one often takes three coats. Set coats to 3 to be safe.
- Textured surfaces use more. Woodchip, textured plaster, or rough masonry can cut coverage by a quarter or more.
- Ceilings count separately. Run the calculator again using the ceiling's length × width as the "wall area" (enter length as total wall length and width as height, with 0 doors and windows).
- Buy the same batch. If you need multiple tins, buy them together and check the batch numbers match — colour can vary slightly between batches.
Typical coverage by paint type
| Paint type | Typical coverage |
|---|---|
| Emulsion / latex (walls & ceilings) | 10–13 m² per litre |
| Kitchen/bathroom paint | 10–12 m² per litre |
| Gloss or satin (wood/metal) | 10–14 m² per litre |
| Masonry paint (smooth render) | 6–10 m² per litre |
| Masonry paint (rough render) | 4–6 m² per litre |
Frequently asked questions
How much paint do I need for an average bedroom?
A typical 3.5 m × 3 m bedroom with 2.4 m ceilings has roughly 31 m² of wall area. After subtracting a door and a window and applying two coats at 12 m²/L, you need about 5 litres — one standard large tin.
Is one coat of paint ever enough?
Only when repainting a wall in the same (or very similar) colour with a good-quality paint. Any colour change, stained walls, or new plaster needs at least two coats.
Should I subtract the skirting boards and radiators?
No — the margin they represent is small, and it's safer to leave them in as your buffer. Only subtract large areas you genuinely won't paint, like fitted wardrobes covering a whole wall.
How much does leftover paint keep?
Sealed properly and stored somewhere cool and frost-free, emulsion keeps 2–5 years. Keep some for touch-ups — it's the same batch, so it will match perfectly.
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