How to calculate how much paint you need for a room

Running out of paint half way up a wall is annoying, and buying far too much is a waste of money. The sum that avoids both is short: work out the area you will paint, multiply by the number of coats, divide by the coverage printed on the tin, and add a little for waste. This guide takes you through each step with a complete worked example for a bedroom, including doors, windows and the ceiling, then explains how coverage changes with the surface, how to choose tin sizes, and what to do about the finishing details such as skirting boards. The numbers are easy to check with a tape measure and a calculator.

By Valtrinity. Updated 18 October 2026.

Step 1: measure the room

You need three measurements: the length of the room, the width, and the height from floor to ceiling. Measure in metres, to the nearest centimetre. A tape measure that locks, or a laser measure, makes this quick, and it is worth measuring each wall separately in an older house, where the walls may not be perfectly square.

Also measure the openings you will not paint: doors and windows, and any large built-in cupboards or fireplaces. A standard UK internal door is about 1.98 metres high and 0.76 metres wide, though sizes vary, so measure yours. Windows vary a lot.

Write the numbers down in a simple list, with the unit, so that you can check them later. A typical bedroom might be 4.2 metres long, 3.6 metres wide and 2.4 metres high.

Step 2: work out the wall area

The four walls together form a rectangle when unfolded. Its length is the perimeter of the room, which is twice the length plus twice the width, and its height is the ceiling height. So wall area = 2 times (length plus width) times height.

For our bedroom, the perimeter is 2 times (4.2 plus 3.6), which is 15.6 metres. Multiply by 2.4 metres and the area is 37.44 square metres.

Now subtract the openings. A door of 1.98 by 0.762 metres is 1.51 square metres. A window of 1.2 by 1.0 metres is 1.2 square metres. Together that is 2.71 square metres. The paintable wall area is 37.44 minus 2.71, which is 34.73 square metres.

If a wall is not a plain rectangle, such as a gable end or a wall under a sloping ceiling, split it into rectangles and triangles, work out each area and add them up. A triangle has an area of half the base times the height.

Step 3: add the ceiling

The ceiling is a plain rectangle: length times width. For our bedroom that is 4.2 times 3.6, which is 15.12 square metres. If you are painting it, keep the ceiling as a separate figure, because it often uses a different paint, such as a flat white ceiling paint, and may need a different number of coats.

If you are not painting the ceiling, skip this step. If the room has a chimney breast, alcoves or a bay window, adjust the measurements to follow the real shape, or use the nearest rectangle for each part and add them up.

Step 4: coats and coverage

Two coats are the normal recommendation for walls, and a darker or a stronger change of colour may need three. Multiply the area by the number of coats. For the walls: 34.73 times 2 is 69.46 square metres of coverage. For the ceiling: 15.12 times 2 is 30.24 square metres.

Next, find the coverage on the tin. Many emulsion paints cover about 10 to 14 square metres per litre for a single coat on a smooth, previously painted surface. The figure is printed on the tin or on the manufacturer's product page. Use that, not a general number, because it varies by paint and by finish. In this example we will use 12 square metres per litre.

Divide the area by the coverage to get litres. Walls: 69.46 divided by 12 is 5.79 litres. Ceiling: 30.24 divided by 12 is 2.52 litres. The total is 8.31 litres.

Step 5: allow for waste and choose tins

Add about 10 per cent for waste, for paint left in the tray and on the roller, and for touch-ups. 8.31 times 1.1 is 9.14 litres. If you are painting the walls only, 5.79 times 1.1 is 6.37 litres.

Paint comes in standard tin sizes, often 1, 2.5, 5 and 10 litres, though exact sizes differ between brands. For the walls alone, 6.37 litres could be bought as a 5 litre tin and a 2.5 litre tin, which is 7.5 litres and leaves a little for touch-ups. For the ceiling, 2.52 litres plus waste is about 2.8 litres, so a 2.5 litre tin plus a 1 litre tin would do.

Buy all the paint for a wall or a room at the same time, and from the same batch if you can. Mix tins together in a bucket before use when the colour is mixed to order, so that any small differences between batches are evened out.

Surface, colour and finish: when the numbers change

New plaster, bare wood and chalky old paint soak up a lot of paint. The first coat on new plaster is often a thinned coat of the same emulsion, called a mist coat, followed by two full coats. In those cases, allow more paint, or follow the maker's guidance for that surface.

Rough or textured surfaces, such as woodchip or Artex, use more paint than smooth ones, because the surface area is larger. Allow an extra 10 to 20 per cent, and check the coverage range on the tin for rough surfaces.

A big change of colour, for instance dark red to white, needs a primer or a stain-blocking undercoat, or an extra coat. A change between similar shades may need no more than two coats. If you are covering a very dark colour, a tinted primer helps.

Woodwork and the small jobs

Skirting boards, door frames and window frames use far less paint than walls, but they usually need different paint, such as satin or gloss, with a different coverage. A 750 ml tin typically covers roughly 10 square metres or more per coat, but check the tin.

For skirting, work out the length in metres and multiply by the height. Our bedroom has a perimeter of 15.6 metres, minus the door width of about 0.8 metres, which is 14.8 metres. With boards 0.1 metres high, the area is 1.48 square metres per coat. Even at three coats, that is 4.4 square metres, well under half a litre.

Add a little for the doors. A standard door has two faces, so it is about 3 square metres per coat, plus the edges. A 750 ml tin usually covers the woodwork in a normal room.

A quick checklist before you buy

Check the room measurements once more, and make sure you subtracted the doors and windows. Check the coverage on the tin of the actual paint you chose, and the number of coats recommended. Compare the total with the sizes on offer, and consider whether buying one larger tin gives you a better price per litre than two smaller ones.

Keep a note of the paint name, colour code and finish. Matching a colour later is much easier with the code, and a small leftover tin, properly sealed, is useful for repairs.

If you are hiring a decorator, the same numbers help you check a quote for materials. Ask what coverage they assume and how many coats.

Tools mentioned in this guide

Frequently asked questions