Paint is sold by volume but used by area, so the number that matters is the spreading rate on the tin. Matt emulsion typically covers 10 to 12 square metres per litre per coat, and this calculator defaults to 11. Textured, porous or freshly plastered surfaces drink noticeably more, and dark colours over light often need a third coat.
The wall area is the room perimeter multiplied by the wall height, less whatever you deduct for doors and windows. A standard internal door is about 1.6 square metres and an average window around 1.5, so five square metres of deduction is a fair starting point for a room with one of each. Ceilings are simply length by width and are handled separately.
For a 4 by 5 metre room with 2.4 metre walls the paintable area is 43.2 square metres before deductions, or 38.2 after five. Two coats at 11 square metres per litre needs 6.95 litres, which in practice means a single 5 litre tin.
The formula
perimeter- Twice the room length plus twice the width, in metres
height- Floor to ceiling height in metres
openings- Area of doors and windows you are not painting, in m²
coverage- Spreading rate from the tin, in m² per litre per coat
How it works, step by step
- Enter the room length, width and wall height in metres.
- Deduct the area of doors and windows you are leaving unpainted.
- Set the number of coats and the coverage figure printed on your tin.
- Read the bare litres on the gauge; tins to buy appears in the breakdown.
Worked examples
A 4 × 5 m bedroom, two coats
Perimeter is 18 m, so walls come to 43.2 m². Deducting 5 m² for a door and window leaves 38.2 m². Two coats at 11 m²/L is 6.95 litres, and with a 10% allowance 7.64 litres — one 5 litre tin.
Fresh plaster changes the answer
New plaster is porous and a mist coat can spread as little as 8 m²/L. The same 38.2 m² then needs 4.77 litres for that first thinned coat alone, on top of 6.95 litres for two finish coats.
How to read your score
Frequently asked questions
Should I include the ceiling?
Not in this figure — enter the ceiling separately as a room whose area is length by width with zero wall height, or simply add length × width × coats ÷ coverage to the result. Ceilings usually take a slightly lower spreading rate because they are rolled overhead.
Why does the tin say 12 m² per litre when I get less?
Manufacturer figures are measured on a smooth, sealed, primed surface with a good roller. Real walls have texture, previous colour showing through, and cutting-in waste. Assuming 10 to 11 rather than 12 keeps you from making a second trip.
How much extra should I allow?
Ten per cent covers roller loss, tray residue and touch-ups. If you are spraying, allow 20 to 30% because overspray and priming the gun waste more than brushwork does.
Do I need a different number for gloss or masonry paint?
Yes. Oil-based gloss and satin on smooth woodwork can reach 14 to 16 m² per litre, while textured masonry paint on render may only manage 6 to 8. Override the coverage field with the figure for your product.
Paint quantity reference
| Room size | Wall area | Bare litres | With 10% waste | 5 L tins |
|---|---|---|---|---|
| 2 × 3 m | 19.0 m² | 3.45 L | 3.80 L | 1 |
| 3 × 3 m | 23.8 m² | 4.33 L | 4.76 L | 1 |
| 3 × 4 m | 28.6 m² | 5.20 L | 5.72 L | 2 |
| 4 × 4 m | 33.4 m² | 6.07 L | 6.68 L | 2 |
| 4 × 5 m | 38.2 m² | 6.95 L | 7.64 L | 2 |
| 5 × 6 m | 47.8 m² | 8.69 L | 9.56 L | 2 |
| 6 × 8 m | 62.2 m² | 11.31 L | 12.44 L | 3 |
Computed at 11 m²/L per coat. Check your tin and override the coverage input.