Paint calculator — how much paint for a room?
Enter your room dimensions and the coverage from the tin. You get the paintable area, the litres you need and how many tins to buy. Nothing is sent anywhere — it all runs in your browser. All figures are metric.
The formula
Paint is calculated from wall area, not floor area — that is the most common mistake. A 20 m² room can have anywhere between 40 and 55 m² of wall depending on its shape and height.
(perimeter × height − openings) × coats ÷ coverage = litres
Worked example for a 4 × 5 m room with 2.7 m walls:
- Perimeter: (4 + 5) × 2 = 18 m
- Walls: 18 × 2.7 = 48.6 m²
- Less a window and a door (approx. 3.3 m²): 45.3 m²
- Two coats: 45.3 × 2 = 90.6 m² to cover
- At 12 m²/l: 90.6 ÷ 12 = 7.6 l → two 5 l tins
Typical paint coverage
Always check the tin — the figures below are for a first estimate before you have picked a product. All of them are for a single coat.
| Type of paint | Coverage (m²/l) | Notes |
|---|---|---|
| Emulsion / latex (walls) | 10–14 | The usual choice, washable |
| Acrylic | 10–12 | Dries fast, low odour |
| Ceramic / stain-resistant | 8–12 | Thicker, lower coverage |
| Ceiling paint (white, matt) | 10–12 | Two coats usually enough |
| Deep-penetrating primer | 8–10 | Single coat, counted separately |
| Wood varnish / enamel | 12–16 | Thin coats, usually 2–3 |
On an absorbent, unprimed surface real coverage can drop by a third. If you are painting fresh plaster, add extra or prime first.
Four mistakes that cost the most
- Using floor area. Wall area is usually about twice the floor area.
- Forgetting the second coat. One coat is only enough when refreshing the same colour.
- Buying exactly enough. Running half a litre short means a second trip and the risk of a different mixing batch.
- Ignoring the colour underneath. Light over dark means three coats or a blocking primer — plan for it upfront.
Would rather not measure by hand? Decoria.Studio scans your room with your phone, works out the wall and floor area itself, then builds a shopping list with quantities and prices — litres of paint included.
Questions and answers
How much paint do I need for a 20 m² room?
With 2.6 m walls and standard openings, roughly 7–8 litres for two coats. What matters is wall area, not floor area: a 20 m² room (4 × 5 m) has about 47 m² of walls, leaving about 43 m² once you subtract a window and a door.
Should I subtract windows and doors?
Yes, if they add up to more than about 3 m². A typical window is around 1.5 m² and a door around 1.8 m². With one window and one door the difference is under a litre, but a fully glazed living room wall changes the result by several litres.
How many coats of paint do I need?
Two coats is the standard and this calculator assumes it. One coat is only enough when refreshing the same colour. Going from a dark shade to a light one, or using a strong colour, plan for three.
What does the coverage on the tin actually mean?
It is the number of square metres one litre covers in a SINGLE coat on a smooth, primed surface. On bare plaster or an absorbent wall the real figure can drop by a third.
Does primer count separately?
Yes. Primer has its own coverage, usually 8–10 m² per litre, and goes on in a single coat. Calculate it separately by entering its coverage and one coat.
Is one large tin better than several small ones?
One large tin. Machine-mixed paint can vary slightly in shade between batches, so a whole room is best painted from a single container. A litre also tends to be cheaper in the larger size.