How the litres are worked out
Litres needed is the area multiplied by the number of coats, divided by the coverage per litre. Forty square metres at two coats and 12 square metres per litre is 6.67 litres.
Tins to buy is that figure divided by the tin size and rounded up, because paint is sold in whole tins. At 5 litre tins, 6.67 litres means two tins.
Buying two 5 litre tins is often cheaper than a 5 and a 2.5, and it leaves paint for touching up.
Worked example
A room with 40 square metres of wall, two coats and 12 square metres per litre needs 6.67 litres, which is two 5 litre tins.
The same room in a strong colour may need three coats, which is 10 litres and still two tins, so the third coat is nearly free in tin terms.
Measuring the area honestly
Measure the wall perimeter and multiply by the height, then subtract the doors and windows. A standard door is about 1.8 square metres and a window about 1.5.
New plaster and bare wood absorb far more than the tin says. Add 20 to 30 per cent for a first coat on a new surface.
Ceilings and woodwork are usually measured separately, and they use different paints with different coverage.
Limits of this calculator
It uses the coverage you enter. A roller on a rough surface covers less than the tin claims, and a sprayer covers more.
It does not include primer, undercoat, or the extra needed for a dramatic colour change.
It does not account for the paint left in the tin, which is rarely usable after a year, or for the cost.
For a papered finish instead, use the wallpaper calculator.
Inputs for the paint calculator
Area to paint
Wall or ceiling area, less the doors and windows you are not painting.
Number of coats
Two coats is normal for a colour change, one for a refresh.
Coverage per litre
The figure on the tin. 12 to 14 square metres per litre is typical for emulsion.
Tin size
Size of the tin you are buying, usually 2.5 or 5 litres.
CALCULATOR QUESTIONS
Paint calculator questions
How much paint for a room?
About 12 litres for a typical room at two coats, which is two and a half 5 litre tins.
How many coats do I need?
Two for a colour change, three for a strong colour over a light one, one for a refresh in the same colour.
Does new plaster need more?
Yes. A mist coat and a further coat absorb 20 to 30 per cent more than the tin suggests.
Is primer included?
No. Primer and undercoat are separate, and bare surfaces usually need them.