Free Wall Area calculator with clear step-by-step results.
Wall area is perimeter times height, less the openings - and deducting openings matters far more here than it does for a ceiling, since doors and windows can easily be 15% of a small room's wall surface. Paint is then sized for two coats at the coverage rate on the tin.
Net wall area
Net = perimeter x height - (doors x door area + windows x window area)
Paint required
Litres = net area x 2 coats / coverage per litre
Deduct anything above about half a square metre. Smaller openings are more than offset by the cutting-in around them.
Optimistic. Quoted coverage assumes a smooth, sealed, previously painted surface. On new plaster or a colour change, plan for 15-20% less.