Paint & Primer Coverage Calculator

How the paint calculation works

Wall area is the room's perimeter times its height: 2 × (length + width) × height. We then deduct 1.9 m² per door and 1.5 m² per window — standard allowances that suit most UK rooms. If you have large patio doors or a full-width window, count them as two windows.

Paint needed is total area × coats ÷ coverage. Coverage depends on the surface: a previously painted wall in a similar colour covers around 12 m² per litre, while new plaster soaks up paint and only manages about 10 m²/L, and covering a dark colour drops to ~9 m²/L.

New plaster needs a mist coat

Fresh plaster is porous and will suck the moisture out of emulsion, causing it to flake. Always start with a mist coat — emulsion thinned with about 10–20% water — before your topcoats. The calculator flags this and estimates the extra paint when you select "New plaster".

Buying the right can sizes

Emulsion is sold in 1L, 2.5L, 5L and 10L cans, and larger cans are better value per litre. The calculator suggests a combination that covers your requirement with minimal leftover — but if you're between sizes, round up: an unopened can keeps for years and is invaluable for touch-ups.

Typical paint coverage rates