How to use the Paint Calculator (Litres Needed for a Room)
- Enter the room length, width and wall height in metres.
- Enter the number of doors and windows to leave out.
- Choose whether to paint the ceiling, and enter the number of coats.
- Enter the coverage in m² per litre from the tin, and read the litres needed.
Formula used
Wall area = 2 × (length + width) × height. Subtract about 1.8 m² per door and 1.5 m² per window. Litres = area × coats ÷ coverage, rounded up to the next half litre.
Worked example
Here is the Paint Calculator (Litres Needed for a Room) worked through with the example values shown above, so you can check each step by hand.
| Inputs | |
|---|---|
| Room length (m) | 5 |
| Room width (m) | 4 |
| Wall height (m) | 2.7 |
| Doors | 1 |
| Windows | 2 |
| Paint the ceiling too? | No, walls only |
| Coats | 2 |
| Coverage of the paint (m² per litre, one coat) | 10 |
| Result | |
| Paint needed (litres, rounded up) | 9 |
| Area to paint (one coat) | 43.8 |
| Wall area before deductions | 48.6 |
How the area is worked out
The four walls of a 5 m by 4 m room that is 2.7 m high have an area of 2 × (5 + 4) × 2.7 = 48.6 m². The calculator subtracts 1.8 m² for each door and 1.5 m² for each window. With one door and two windows the paintable area is 43.8 m².
If you also paint the ceiling, the room’s floor area is added, which makes the area 63.8 m² in this example.
From area to litres
Litres = area × coats ÷ coverage. With two coats and a coverage of 10 m² per litre, the example needs 8.76 litres, which the calculator rounds up to the next half litre: 9 litres. One coat would need 4.5 litres.
A bigger room (8 m by 6 m and 3 m high) needs 16 litres for walls with the same settings.
Choosing the coverage
Use the coverage printed on the tin, which is often between 8 and 12 m² per litre for one coat of emulsion. Porous or rough walls soak up more paint, and a deep colour over a light one, or the reverse, often needs extra coats.
A primer or undercoat is a separate product with its own coverage, so calculate it with another run if you need it.
Practical tips
Add a little extra for touch-ups, and buy all the paint for a room at the same time, from the same batch, so the colour matches. Check how the litres fit into the tin sizes sold, for example 1, 4 and 10 litres, and buy the combination that comes closest above the number shown.
The calculator treats the room as a plain box. Skirting boards, alcoves and textured finishes change the real figure slightly, so allow some margin.
Frequently asked questions
How many coats do I need?
Two is typical. A single coat can work when painting over a similar colour with good paint.
What area do doors and windows take off?
The calculator uses about 1.8 m² per door and 1.5 m² per window.
Does it include the ceiling?
Only if you choose walls and ceiling.
Can I enter feet?
This calculator uses metres. Convert feet by multiplying by 0.3048.
What if I need primer?
Run it again with the primer’s coverage and number of coats.
Why does it round up?
It is better to have a little extra than run out partway through a wall.
This calculator gives estimates for information only. Actual figures from a bank, employer, tax authority or doctor may differ. It is not financial, tax, legal or medical advice.