Measure the walls
Add the lengths of all the walls to get the perimeter and multiply by the height. A room 4 m × 3.5 m with walls 2.5 m high has 2 × (4 + 3.5) × 2.5 = 37.5 m² of wall.
Take off doors and windows
A standard door is about 1.9 m² (21 ft²) and a window about 1.4 m² (15 ft²). With one door and two windows the room above has 37.5 − 1.9 − 2 × 1.4 = 32.8 m² to paint.
Coats and coverage
The tin gives the coverage, often about 10 m² per litre or 350 to 400 ft² per US gallon. Two coats are normal, and more are needed for a big colour change or bare plaster.
32.8 m² × 2 coats ÷ 10 m²/L = 6.6 litres. Buying a 5 L and a 2.5 L tin covers it with a little spare for touch-ups.
Tips
- Measure the ceiling separately (length × width) if you are painting it too.
- Rough or porous surfaces absorb more paint than the tin says.
- Buy all the paint for one room at once so the colour matches.
Calculator
The Paint Calculator does the whole calculation in metres or feet. For the floor, the Flooring Calculator works out the boxes to buy.