Paint Calculator
Buying one tin short halfway through a wall is worse than buying one too many — colour batches vary.
How the estimate is built
Standard allowances: an internal door is roughly 1.8 m² (0.76 × 1.98 m) and an average window about 1.5 m². Measure large windows, patio doors or bay windows individually — they change the answer substantially.
Coverage varies by paint type. Matt emulsion typically covers 12–14 m² per litre, silk slightly less, and gloss and specialist paints as little as 10. The tin states its own figure; use it rather than the default.
How many coats you really need
| Situation | Coats |
|---|---|
| Same colour, good condition | 1 |
| Similar shade, previously painted | 2 |
| Light over dark | 3, or primer plus 2 |
| Strong colour (red, yellow) over light | 3, sometimes 4 |
| New plaster | Mist coat plus 2 |
New plaster needs a mist coat — emulsion thinned with about 10 % water — before anything else. Fresh plaster is porous, and full-strength paint applied directly sits on the surface and peels away later.
Reds and yellows genuinely need more coats than other colours. Their pigments are less opaque, so an extra coat is normal rather than a sign of poor application.
Buying and storing
Always round up, and buy a little more than the calculation says. Paint from different manufacturing batches can differ subtly in shade, and finding this out with one wall left to do is a genuinely annoying problem. Tins carry a batch number — buying all of them together avoids it entirely.
Practical notes:
- A 5-litre tin covers roughly 60 m² in one coat, which is around a 4 × 4 m room's walls.
- Keep leftover paint for touch-ups; sealed properly it lasts years. Note the room on the lid.
- Add a 10 % contingency for textured surfaces, which absorb noticeably more.
- Do not pour leftover paint down a drain. Most local authorities have a disposal route.
Frequently asked questions
Should I deduct for doors and windows?
Yes for anything substantial. Skipping the deduction on a room with two large windows and a door means buying roughly 10 % more paint than needed. For one small window the difference is within the contingency anyway.
Does primer count as a coat?
No — it is a separate product with its own coverage, usually similar to emulsion. Calculate it separately at one coat over the same area. Primer is needed on bare plaster, bare wood, metal, and when covering stains or a very dark colour.
How much does a ceiling add?
For a typical room, roughly a third to a half of the wall area. A 4.5 × 3.6 m room has about 39 m² of wall and 16 m² of ceiling. Ceilings often need two coats regardless of colour, because they show unevenness badly.