Walls and ceiling together, before deductions — those come below.
It is on the tin. Allow less on a porous or previously unpainted surface.
How this calculation works
Why the coverage on the tin rarely holds
The figure assumes an ideal surface. In practice fresh plaster drinks it, old emulsion takes it unevenly, and going from a dark colour to a light one costs you an extra coat. Work with the coverage you know from your own jobs rather than the one on the label.
Why one tin more is cheaper
A second trip to the merchant costs an hour. A tin costs less than an hour.
Frequently asked questions
Does this include primer?
No — put primer in as its own calculation, because the coverage and the number of coats are different. On bare plaster a mist coat is usually the first of those.
What about ceilings?
Add the ceiling area to the wall area. If you are using a different paint on it, run the calculation twice.
Why round up to whole tins?
Because you buy tins, not litres, and because two batches of the same colour are not always the same colour.
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