Step one: measure the area
For walls, measure the length of each wall and the height from skirting to ceiling, then multiply to get the area in square metres. Add the walls together. You can subtract large windows and doors, but many decorators don't bother – the extra paint is useful for cutting in and touch-ups.
For a ceiling, simply multiply the length of the room by its width.
| Surface | Calculation | Area |
|---|---|---|
| Two long walls | 2 × 4m × 2.4m | 19.2 m² |
| Two short walls | 2 × 3.5m × 2.4m | 16.8 m² |
| Walls total | 36 m² | |
| Ceiling | 4m × 3.5m | 14 m² |
Example: a 4m × 3.5m bedroom with 2.4m ceilings
Step two: check the coverage on the tin
Every paint lists its coverage, usually in square metres per litre per coat. Many interior emulsions sit somewhere around 10–14 m² per litre on smooth, previously painted walls, but it varies by product. New plaster, rough surfaces and textured walls soak up much more.
Divide the area by the coverage, then multiply by the number of coats. In the example above, 36 m² of walls at 12 m² per litre is 3 litres per coat – so around 6 litres for two coats.
Why you'll probably need more
Calculations assume perfect conditions. In real rooms you'll lose some to rollers and trays, and certain jobs need extra:
- Strong or dark colours – often an extra coat
- Going from dark to light – may need an extra coat or a primer
- New plaster – the mist coat soaks in heavily
- Rough or textured surfaces – much lower coverage
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Woodwork
Trim paint goes a long way. A 750ml tin often covers the skirting, door and frame of an average room in one coat, but check the coverage on the tin and allow for undercoat on bare or very dark woodwork.
If you're hiring a decorator
Most decorators supply paint and work out the quantities themselves, and the cost is included in the quote. If you'd rather buy the paint yourself – perhaps a particular designer brand – ask them how much to get. They'll know how their chosen product behaves on your walls.
