How to work out how much paint you need
- Measure the length and width of the room and the height of the walls, and choose metric or imperial units.
- Enter the number of doors and windows; standard sizes are subtracted automatically.
- Choose the number of coats and tick the ceiling if you are painting it.
- Check the coverage printed on your paint tin and enter it. The result shows litres or gallons and the tins to buy.
How the paint calculation works
The wall area is the room's perimeter multiplied by the wall height: for a 4 × 3.5 m room with 2.5 m walls that is 2 × (4 + 3.5) × 2.5 = 37.5 m². A standard door (about 1.9 m² or 21 sq ft) and window (about 1.5 m² or 15 sq ft) are subtracted. Multiply by the number of coats and divide by the coverage, then add 10% for touch-ups, textured walls and paint left in the tray. With two coats at 10 m² per litre, that room needs about 7.5 litres, and the calculator suggests the combination of tins with the least left over (here one 5 L tin and three 1 L tins).
Coverage varies by product and surface. Most interior emulsions cover 10–14 m² per litre (350–400 sq ft per US gallon) on smooth, previously painted walls. Fresh plaster, bare wood, rough or porous surfaces and big colour changes need a primer or an extra coat; dark colours over light ones, and light over dark, often need three. Always buy the paint for one room from the same batch to avoid slight colour differences. The calculator runs in your browser.
How many coats of paint do I need?
Two coats for most walls. One may do for a fresh coat of the same colour; three for strong colour changes.
How much paint for a ceiling?
The ceiling area is length × width. Tick "Include the ceiling", or use a separate ceiling paint and calculate it alone.
Should I subtract doors and windows?
Yes, for accuracy; but the 10% margin means small openings do not matter much.