Every other calculator tells you 14.6 litres. Nobody sells 14.6 litres. This one tells you which tins to put in the trolley, exactly how much you'll have left over — and if you've already got enough in the shed, it tells you to buy nothing.
Walls, ceilings, woodwork and exteriors. Enter the room, pick the paint, and tell us the tin sizes your shop stocks — we'll work out the combinations.
Works in metric and US imperial — you choose at the start and can switch at any time. Every figure this calculator uses is published with its source, and the result comes with a printable order sheet. No account, no email, and nothing you type leaves your browser.
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Walls are perimeter × height minus the openings; ceilings and floors are plain area. That surface, divided by the spread rate and multiplied by the number of coats, is the litres or gallons you need — and then comes the part every other calculator skips: nobody sells 14.6 litres. The calculator works out which combination of the tin sizes your shop stocks covers the job with the least left over.
Check the shed first — skin on top is fine once stirred and strained, but gritty, lumpy or sour-smelling paint is gone. Then read the tin's own spread rate rather than trusting a website default. Then buy the whole job in one go, from one batch. Ceilings are usually a separate tin, so run the ceiling as its own calculation rather than adding it to the walls.
Rough render is a paint sponge — a first coat on unpainted render can swallow half again as much as the label claims, so budget three coats. Going light over dark also means three coats, or a tinted primer first. The calculator asks about both, because they are the two places a tidy sum goes wrong.
Wall area is perimeter × height, minus doors and windows — usually two to three times the floor area, which is why floor area is the wrong starting point. Divide by the spread rate on your tin, multiply by coats, then round up to tins you can actually buy. This calculator does the tin arithmetic for the sizes your shop stocks, and if what is in the shed already covers it, it tells you to buy nothing.
If your tin states a spread rate, use it — the label beats any calculator's default. Defaults here are deliberately cautious so you don't run short, and UK and US labels quote differently, which is optimism rather than chemistry.
Budget three coats, or a tinted primer first. Two coats of white over a deep colour will ghost. Unpainted rough render is its own case: the first coat can swallow half again what the label claims.
Tins mixed in different batches can differ slightly in shade. Run short, go back next week, and the patch may show. This is the main reason to round up rather than down — and to label the leftovers with room and date, because two years from now that tin is the difference between a touch-up and repainting the wall.