How the paint calculator works
To work out how much paint you need, the calculator finds the area of your walls, takes off doors and windows, multiplies by the number of coats and divides by how far a litre of paint spreads. The sums are:
wall area = 2 × (length + width) × height
area to paint = (wall area − doors − windows + ceiling) × coats
litres = area to paint ÷ coverage per litre
It then rounds to the nearest 0.1 litre and suggests the cheapest mix of 2.5L and 5L tins that covers it, along with how many tins you'd need if your shop only sells one size.
What coverage figure should I use?
We start at 12m² per litre, which is typical for a standard matt or silk emulsion on smooth, previously painted plaster. The figure printed on the tin is the best guide, and most brands quote somewhere between 10 and 13m² per litre. Use a lower number when:
- Painting new plaster. Bare plaster soaks paint up, so the first coat (a mist coat of watered-down emulsion) goes much less far. Try 8 to 9m² per litre.
- Covering a strong colour. Going from dark red to white may need three coats rather than two.
- Using a textured roller or rough surface, such as Artex or woodchip paper, which has more surface area than its flat size.
- Using a one-coat paint. These are thicker and often cover only 8 to 10m² per litre, so check the tin.
Worked example: a 4m × 3.5m bedroom
Take a bedroom 4m long and 3.5m wide with 2.4m high walls, one door and one window of 1.5m².
- Distance around the room: 2 × (4 + 3.5) = 15m
- Wall area: 15 × 2.4 = 36m²
- Take off the door (1.5m²) and window (1.5m²): 36 − 3 = 33m²
- Two coats: 33 × 2 = 66m²
- Paint needed: 66 ÷ 12 = 5.5 litres
The best buy is one 5L tin and one 2.5L tin. If you add the 14m² ceiling with the same paint, you need 7.8 litres, so two 5L tins.
Paint needed for common room sizes
Two coats on the walls only, 2.4m ceilings, one door and one window, at 12m² per litre:
| Room | Wall area to paint | Paint needed |
|---|---|---|
| Small bathroom, 2m × 2m | 16.2m² | 2.7 litres |
| Box room, 3m × 2.5m | 23.4m² | 3.9 litres |
| Double bedroom, 4m × 3.5m | 33m² | 5.5 litres |
| Living room, 5m × 4m | 40.2m² | 6.7 litres |
| Large lounge, 6m × 5m | 49.8m² | 8.3 litres |
Tips for buying and using paint
- Buy a little extra. Batches can vary slightly in colour, so it's better to have a bit left over for touch-ups than to run out halfway through a wall.
- Two 2.5L tins often cost more than one 5L tin. Compare the price per litre before you buy.
- Measure the walls, not the floor, if the room has a chimney breast or alcoves. Add the extra wall faces as a longer length.
- Woodwork and radiators need different paint. Skirting boards and door frames take gloss or satinwood, usually sold in 750ml or 2.5L tins, so work those out separately using the coverage on the tin.
- Ceilings are often painted first, then the walls, so any drips on the walls get covered.
Doing up the whole room? Work out how many packs of laminate or carpet tiles you need with the flooring calculator, or use the tile calculator for a bathroom or kitchen splashback.
Frequently asked questions
How much paint do I need for a 4m × 4m room?
With 2.4m walls, one door and one window, the walls come to about 35.4m². Two coats at 12m² per litre needs about 5.9 litres, so one 5L tin plus one 2.5L tin.
How many coats of paint should I do?
Two coats is the norm for walls. Use three if you're covering a much darker colour, and a mist coat plus two full coats on new plaster.
How far does a 2.5 litre tin of paint go?
At a typical 12m² per litre, a 2.5L tin covers about 30m² with one coat, or 15m² with two coats. That's roughly one wall of a large room or a small bathroom.
Should I take off doors and windows?
Yes. A standard internal door is about 1.5m² and an average window about 1.5m². In a small room with several doors, leaving them in could mean buying an extra tin you don't need.
Is one-coat paint really one coat?
Usually only when you're painting over a similar colour on a sound surface. One-coat paints are thicker, so they tend to cover less area per litre than standard emulsion.
Last reviewed October 2026 using 2026/27 figures. This calculator gives an estimate for guidance only.