Paint Calculator

How the paint quantity is calculated

Wall area is the room's perimeter multiplied by its height, minus 1.8 square metres for every door and 1.2 for every window. So a 4 by 5 metre room at 2.4 metres high has a 18 metre perimeter and 43.2 square metres of wall; one door and two windows take that to 39 square metres. Litres are then wall area multiplied by the number of coats and by a surface factor, divided by the coverage of the paint you picked: 9 square metres a litre for budget, 11 for standard, 13 for premium. The surface factor is 1.0 for smooth or previously painted, 1.3 for textured and 1.5 for raw plaster — raw plaster drinks half as much again, which is the argument for a mist coat before the expensive tin. Ceilings, if you tick them, add width times length at the same coverage.

The total is then multiplied by 1.1. That ten per cent is for what stays in the tray, the roller and the brush, for touch-ups, and for the fact that nobody measures a room perfectly; running out three-quarters of the way up a wall costs far more than a part-used tin. Cost is the buffered litres times the price per litre you entered.

The shopping list is a greedy fill from the largest tin down — 10, 5, 2.5 and 1 litre — and then rounds the remainder up to the smallest tin that covers it. It optimises for not overbuying rather than for the lowest price per litre, so on a large job a bigger tin may still work out cheaper than the combination it suggests. The door and window allowances, the three coverage bands and the surface factors are our own working figures. Coverage in particular is printed on the tin you are actually buying, and that number beats ours.

Every figure above was read out of the code that runs on this page. Where a number is our own judgement rather than a published one, the note says so and cites nothing. Method last checked 21 August 2026.