Paint Coverage Calculator

The formula this calculator uses

Litres needed = (wall area x coats) / coverage per litre

A

Wall area to be painted, in square metres

c

Number of coats, usually two

R

Coverage per litre per coat, as printed on the tin

S

Tin size, commonly 5 litres or 10 litres

How to check the result by hand

1. 1

   Measure the total paintable area

   Multiply each wall's length by its height and add the rooms together. A 5 m by 4 m room with a 2.5 m ceiling gives an 18 m perimeter times 2.5 m, or 45 m2; four such rooms give 180 m2. Subtract only large openings such as windows and doors.

2. 2

   Multiply by the number of coats

   Two coats on 120 m2 of wall give 240 m2 of painted surface. The coat count is the multiplier that comes first, before any division, because coverage is always quoted for a single coat.

3. 3

   Divide by the coverage per litre

   240 m2 divided by 12 m2 per litre gives 20.0 litres. Keep the coverage figure for the actual product and surface, since a porous first coat can drop the effective rate well below the tin's headline number.

4. 4

   Convert litres into whole tins

   20.0 litres is 4.0 five-litre tins or 2.0 ten-litre tins. Tins are fixed sizes, so round up to the next whole tin and accept the surplus rather than planning around a fractional tin that does not exist.

5. 5

   Add a margin for touch-ups and porosity

   Round the purchase up rather than down: 25.0 litres covers the same 120 m2 with room for a third coat on a stubborn patch, and opened paint rarely keeps well enough to be relied on for repairs months later.