How many litres of paint to buy? Working out area and coverage

To know how many litres of paint to buy, work out the area to paint in square metres and divide it by the paint's coverage, which is shown on the tin (in m² per litre). Then multiply by the number of coats, usually two. Add a small margin for touch-ups. Let us see how it is done, step by step.

Step 1: measure the walls

You need a tape measure. Measure the length of each wall and the height from floor to ceiling.

Multiply the length by the height of each wall. The result is the area in m².
Add up the areas of all the walls.
Measure the doors and windows and work out the area of each one.
Subtract those areas from the total for the walls.
If you are going to paint the ceiling, measure the length and width of the room and multiply.

A simple example: a wall 4 metres long and 2.5 metres high is 10 m². If it has a door of about 2 m², the area to paint is 8 m².

Step 2: check the coverage

Coverage tells you how many square metres one litre of paint covers, in one coat. It is written on the tin or in the technical data sheet. It may be given as a range, for example between one figure and another per litre.

It can vary quite a lot. Many water-based wall paints cover somewhere around ten square metres per litre, per coat, but always check the tin of the paint you have chosen. The real figure depends on the paint and the surface.

The coverage on the tin is an ideal figure, on a smooth, prepared surface. In practice, it covers less.

Step 3: do the sum

Divide the area to paint by the coverage and multiply by the number of coats.

Area to paint: for example, 40 m².
Coverage: for example, 10 m² per litre.
40 divided by 10 gives 4 litres per coat.
For two coats, that is 8 litres.

These numbers are only an example. Use the figures for your room and your paint.

What makes you use more paint

Some situations increase consumption. Allow for them when you do the sum.

Wall with rough or very porous plaster: absorbs more paint.
Changing from a dark colour to a light one: may need more coats.
New wall or one without primer: soaks up more paint on the first coat.
Heavily thinned paint: covers less.
Rolling without loading the roller properly: uses more to cover less.

Margin and tin sizes

Buy a little more than the sum gives. A small margin lets you do touch-ups and cover the unexpected. Keeping a small amount for the future is useful, because the same colour bought later may have slight differences in shade.

Buy all the paint of the same colour at the same time. If you need several tins, mix them in a large bucket before painting. That avoids differences in shade between tins.

Also check the tin sizes available. Sometimes two medium-sized tins make more sense than one very large and one very small.

Tip: Even with the sum done, ask for help at the paint shop. Tell them the type of wall, the current colour and the new one. People who work with paint are usually good at estimating coats.