Paint stores sell by the litre or gallon, but your walls are measured in square metres. Converting correctly prevents leftover waste and mid-job shortages.
Basic formula
Wall area = 2 × (Length + Width) × Height − Door/window areas
Paint needed (litres) = (Wall area × Number of coats) ÷ Coverage per litre
Typical coverage for quality emulsion is about 10–12 m² per litre per coat on smooth plaster. Textured or porous surfaces may drop to 6–8 m²/L.
Ceilings and trim
Ceilings are Length × Width. Trim and doors often need a different product—estimate them separately rather than mixing wall emulsion coverage rates.
Coats matter
- New plaster: primer + 2 finish coats is common.
- Colour change (dark → light): expect an extra coat.
- Always round up to the next practical tin size.
Plug your room sizes into the Paint Calculator for a quick litre estimate before you buy.