Paint calculator

Guess the paint and you either make a second trip to the merchant mid-job or end up with tins you'll never open again. This calculator works out how much you need for a room, in litres and tins, from the wall size, the coats and the paint's coverage. Here's how it's worked out.

How to work out how much paint you need

It comes down to three things: how much wall there is, how many coats it's getting, and how far the paint goes.

Litres = wall area × coats ÷ coverage.

So a room with 28.8 square metres of wall, getting two coats of a paint that covers 12 square metres per litre, needs 28.8 × 2 ÷ 12 = 4.8 litres. The calculator measures the walls from the room size for you, so you only need to know the dimensions.

Measuring the walls

The wall area of a room is the distance around it times the height. Add the length and the width, double it for all four walls, and multiply by the wall height.

Wall area = 2 × (length + width) × height.

A 4m by 3m room with 2.4m walls is 2 × (4 + 3) × 2.4 = 33.6 square metres before you take off the doors and windows.

Deducting doors and windows

Painting the door and the glass isn't happening, so take those areas off. As a rule of thumb, the calculator deducts about 1.8 square metres for each door and 1.5 square metres for each window. If your openings are unusually large, like patio doors or a bay window, adjust the figures so the estimate stays honest. On our example room, one door and two windows take off 4.8 square metres, leaving 28.8 to paint.

How many coats?

Two coats is the standard for a solid, even finish, and it's what the calculator defaults to. You'll want more in a couple of cases:

• New plaster needs a thinned mist coat first to seal it, then two full coats, so buy for three passes.
• Strong colour changes, like white over a deep red, often need three.
• Bold or pale colours can be poor coverers and need an extra coat whatever's underneath.

Coverage varies, so check the tin

The coverage figure is where estimates drift. Most emulsion claims 10 to 14 square metres per litre, but a smooth, previously painted wall gets far more out of a tin than bare or textured plaster, which drinks it. The number on the tin is a best case on a good surface, so lean to the lower end for anything rough or absorbent. That's exactly what the coverage field is for: put in the real figure for your paint and surface.

Ceilings and woodwork

This works out the walls. If you're doing the ceiling too, its area is simply the room length times the width, so a 4m by 3m room is 12 square metres of ceiling, usually in a dedicated ceiling paint. Skirting, frames and doors use woodwork paint and go a long way, so they're normally estimated separately rather than lumped in with the walls.

Buy whole tins, with a little spare

Paint comes in tins, so round up. A drop left over is cheap insurance for touch-ups later, especially in a mixed colour you might struggle to match. Running out with one wall to go, and ending up with a visible join where the new tin starts, is the outcome worth paying a few pounds to avoid.

From materials to a priced job

Once you know the paint, the rest of the job is labour and a fair margin. Drop your materials into a clean, itemised quote with the quote generator, and make sure the whole job pays with how to price a job so you actually make a profit. Doing groundwork on the same project? The concrete calculator handles slabs and post holes.

This tool is for general estimating and gives approximate quantities. Coverage, surfaces and colours vary, so check the coverage on your tin and buy a sensible amount over rather than exactly to the calculation.