How to use this calculator
- Enter the room's length, width and wall height.
- Enter how many doors and windows there are and their size.
- Choose the number of coats and check the coverage printed on your tin.
- Tick "Paint the ceiling too" if you are doing the ceiling in the same paint.
The formula
Wall area = 2 × (length + width) × height − doors − windows
Paint = area × coats ÷ coverage
Tins = paint ÷ tin size, rounded up
Paint makers quote coverage for one coat on a smooth, previously painted wall — typically 10–14 m² per litre for emulsion, or 350–400 ft² per US gallon. We start at 10 m²/L and 350 ft²/gal so you are less likely to run short.
Worked example
A 4 m × 3 m room with 2.5 m walls, one door and one window has 31.6 m² of wall.
Two coats at 10 m²/L needs 6.32 L — 3 tins of 2.5 L.
Tips before you buy
- Buy all the tins at once and mix them together ("boxing") so the colour is even.
- Dark colours over light walls, or light over dark, often need an extra coat or a tinted primer.
- Keep a small amount for touch-ups, labelled with the room and colour code.
Frequently asked questions
›How much does a litre of paint cover?
Usually 10–14 m² per coat on smooth walls, less on new plaster or rough surfaces. Always check the tin.
›How much does a gallon of paint cover?
About 350–400 ft² per coat on smooth, primed walls.
›Should I subtract doors and windows?
Yes for large openings. For a small room the difference is minor, and a little extra paint is handy for touch-ups.
›Do I need a primer?
On new plaster or plasterboard, use a mist coat or primer. It seals the surface so your topcoats cover as the tin says.
Sources
- Coverage ranges: paint manufacturers' published figures for emulsion and interior latex paint.
Formulas last reviewed: September 29, 2026. How we check our formulas