Paint is sold by the litre and spread by the square metre. The tin prints a coverage in square metres per litre, and that is the number that decides the count.
Wall area
Add up the widths of the walls, multiply by the height, and take off doors and windows. A 3 m by 4 m room is 14 m round. At 2.4 m high that is 33.6 m², and 30.8 m² once a door and a window, about 2.8 m² together, are taken off.
Litres
Litres are the area times the number of coats, divided by the coverage. Baglist starts at 12 m² per litre per coat, a common figure for interior emulsion but not a promise. The tin wins.
Two coats over 30.8 m² at 12 m² per litre is 5.13 litres.
| Room | Wall area | Litres |
|---|---|---|
| 3 × 3 m | 26.0 m² | 4.33 |
| 3 × 4 m | 30.8 m² | 5.13 |
| 4 × 5 m | 40.4 m² | 6.73 |
Ceilings
A ceiling is the room's length times its width, with nothing taken off. A 3 × 3 m ceiling is 9 m², and two coats at 12 m² per litre is 1.5 litres. A 4 × 5 m ceiling is 3.33 litres.
Choosing tins
Dulux Trade in the UK lists 2.5 L, 5 L and 10 L. Bunnings lists Dulux Wash&Wear in 1 L, 4 L and 10 L. The calculator shows how many of each size and what is left over.
For 5.13 litres in the UK, one 10 L tin leaves 4.87 L over, two 5 L tins leave the same, and three 2.5 L tins leave 2.37 L. A new colour over a dark one, or bare plaster, usually takes an extra coat or a primer.
Figures are estimates for a shopping list. Check the pack or datasheet before you buy.