How much paint does the room need?
Wall area less the openings, times the coats, divided by the coverage on the tin.
2 tins
4.39 litres for 30.7 square metres of wall.
- 14 m around the room by 2.4 m high is 33.6 square metres of wall.
- 1 doors at 1.5 and 1 windows at 1.4 square metres come to 2.9, leaving 30.7 to paint.
- 30.7 square metres over 2 coats at 14 square metres a litre is 4.39 litres.
- 4.39 litres divided by a 2.5 litre tin is 2 tins, rounded up.
Working it out on paper
The sum is short enough to do by hand, and it is the same sum the tool does.
- Add up the four walls to get the distance around the room, then multiply by the height.
- Take off 1.5 square metres for each door and 1.4 for each window.
- Multiply what is left by the number of coats.
- Divide by the coverage on the tin in square metres a litre, then divide by the tin size and round up.
| Room | Around the room | Wall at 2.4 m | Two coats |
|---|---|---|---|
| Small bedroom, 3 by 3 | 12 m | 28.8 m² | 4.1 L |
| Bedroom, 4 by 3 | 14 m | 33.6 m² | 4.8 L |
| Living room, 5 by 4 | 18 m | 43.2 m² | 6.2 L |
| Large room, 6 by 4 | 20 m | 48.0 m² | 6.9 L |
| Hall and landing, about | 24 m | 57.6 m² | 8.2 L |
Coverage is the number that matters
Everything else in this calculation is measurement, and coverage is the one figure that changes the answer by half. It is printed on every tin, usually as square metres per litre, and it varies more than people expect. A matt emulsion on a smooth previously painted wall will give you around fourteen square metres a litre. Eggshell, satin and gloss are nearer twelve. Exterior masonry paint is around eight, and a textured masonry paint or bare plaster can be six or less, because the surface is drinking it.
Those figures are single coat figures. Every manufacturer quotes them that way and it is the commonest reason a job runs out of paint on the second coat. This tool multiplies by the number of coats before it divides, which is the right order and the order most mental arithmetic gets wrong.
Measuring the room
Add up the length of all four walls to get the distance around the room, then multiply by the height. That gives you the total wall area including the openings, which is why the next step takes them off. A door is about one and a half square metres and a window about one and four tenths, so a room with a door and two windows loses about four and a third square metres, which at two coats and fourteen square metres a litre is around two thirds of a litre. On a small job that is the difference between buying a two and a half litre tin and a five.
Ceilings are a separate sum: length times width, and the same coverage figure applies. If you are painting the ceiling in the same colour, work it out and add it in rather than hoping the wall figure stretches.
Buying it
Round up to whole tins, which the calculator does, and think about tin sizes rather than litres. Two and a half litres and five litres are the common ones, and two two and a half litre tins usually cost more than one five litre tin. Buy the whole job in one go if you can, because paint mixed at different times can differ very slightly in shade, and the difference shows on a long wall in daylight.
Keep whatever is left over, labelled with the room and the date. A tin with two centimetres in the bottom is worth more in three years, when something gets scuffed, than the twenty minutes it takes to find a match.
Questions from the margin
How much wall does a litre of paint cover?
About 14 square metres a coat for matt emulsion on a smooth wall, about 12 for eggshell or gloss, and about 8 for exterior masonry paint. Textured or bare surfaces drink far more. The figure on the tin is the one to use.
How many coats do I need?
Two, almost always. One coat over the same colour can be enough, and a dramatic colour change over a dark wall can want three plus a primer. The calculator multiplies by whatever number you give it.
Do I really need to subtract the windows and doors?
For one small window it hardly matters. For a room with french doors and two large windows it can be six or seven square metres, which is half a litre a coat, which is the difference between two tins and three.
What size is a standard door?
This tool uses 1.5 square metres for a door and 1.4 for a window, which are close to the usual British sizes. If yours are very different, measure them and take the total off the wall area yourself.