Paint Calculator — How Much Paint You Need

How much paint to buy

Enter your wall perimeter and height, the number of doors and windows to subtract, and the number of coats. The tool works out the paintable area and how much paint that needs at the coverage rate (default ~350 sq ft per gallon or ~10 m² per litre), then rounds up to whole cans.

Keep the defaults honest

Real coverage varies with the surface, colour change and primer, so the coverage is an editable default, not a fixed truth. Doors and windows come off at a fixed size — 21 ft² and 15 ft² each, or 1.9 m² and 1.4 m² in metric — printed on their labels. When in doubt, buy a little extra for touch-ups. Everything is calculated in your browser.

Coverage is the number that decides everything

The figure on the tin is measured on a smooth, sealed, previously painted wall. Bare plaster, new plasterboard, render, textured paper and anything porous will drink noticeably more — a first coat on new plaster can take half again as much as the tin claims. Colour matters too: a strong pigment over a pale base covers in two coats, a pale one over a strong base can need three or four. The coverage field is editable because your surface, not the tin, sets the real number, and the tin is where to start rather than where to finish.

Openings, coats and the tin you should buy anyway

Subtracting doors and windows is right for large openings and pointless for small ones — the paint you save on a window is roughly the paint you lose cutting in around it. Two coats is the normal answer; one coat only works when you are repainting the same colour on a sound surface. Priming a bare or heavily patched wall costs a coat but saves more than a coat of finish. Whatever the arithmetic says, buy in whole tins with a margin: touching up later with a second batch of a mixed colour rarely matches, and paint keeps for years.