How much paint do I need? Litres and tins for your room

Type your room's length, width and ceiling height. The calculator works out the area to paint, the litres for your coats, and the cheapest mix of 5 l and 2.5 l tins. Every number it uses is shown under the answer.

The formula, written out

Paint is sold by the litre, and every tin says how many square metres a litre covers. So you need the area, the number of coats and the tin's coverage:

For the ceiling, multiply the length by the width: 4 × 3 = 12 m², so 12 × 2 ÷ 13 = 1.8 litres. Paint both and you need 6.6 litres: a 5 l tin and a 2.5 l tin.

Shops then add a little. B&Q and Homebase both suggest about 10% extra for spills, mistakes and touch-ups. The calculator adds it when "Spare" is on, and tells you when the tins would cover the job without it.

Paint for common UK rooms

Each row is a room with 2.4 m ceilings, one door and one window, two coats of matt emulsion at 13 m² a litre. The tins include the 10% spare.

A feature wall needs far less. One 4 m wall, 2.4 m high, is 9.6 m²: 1.5 litres for two coats, so one 2.5 l tin. Dulux suggests a 1 l tin for a 2.5 m feature wall. A 2.6 m ceiling adds about 8% to the wall figures, so type your own height in the calculator.

How much a litre covers

Coverage is the number that changes the answer most. Paint makers measure it on a smooth, sealed wall. Here is what the main UK sources say:

For the 4 × 3 m room, 13 m² a litre gives 4.7 litres and 10 m² a litre gives 6.1 litres. That is the gap between one tin and two. Use the figure on your own tin: choose "Other" in the calculator and type it in. On textured walls, bare plaster or a big colour change, expect less than the tin says.

The cheapest mix of tins

Emulsion comes in 2.5 l and 5 l tins, and some colours in 1 l. The bigger tin is cheaper per litre. At B&Q on 31 October 2026, Dulux Walls & Ceilings matt cost:

• 5 l: £36, £7.20 a litre.
• 2.5 l: £25, £10 a litre.

So two 2.5 l tins (£50) cost more than one 5 l tin (£36). The calculator fills the job with 5 l tins first. If what is left fits in a 2.5 l tin, it adds one; if not, it adds another 5 l. Prices and multi-buy offers change, so check the price per litre on the shelf.

New plaster: the mist coat

Fresh plaster is dry and soaks up paint. Full-strength emulsion on it can fail to stick, so decorators first put on a mist coat: emulsion thinned with water, which soaks in and seals the surface.

• Wait until the plaster has dried from dark brown to an even pale pink. B&Q says that usually takes at least a week, and can take several.
• Thin it two parts emulsion to one part water (B&Q). Other guides give 3:1 or 1:1. The tin's own instructions come first.
• Count it as a coat. No paint maker gives litres for a mist coat. The calculator counts it as one more full coat: the water stretches the paint, and the plaster soaks it up.

Choose "New plaster" in the calculator and it adds the mist coat, and tells you how much of the total it is.

How to measure a room for paint

1. Measure each wall's width at waist height, corner to corner. For a plain box room, the length and the width are enough.
2. Measure the height from the floor to the ceiling. Do not take off the skirting: the small extra covers touch-ups.
3. Count the doors and windows. A standard UK internal door is 762 × 1981 mm, about 1.5 m² (Howarth Timber). Measure your windows, glass and frame; the calculator starts at 120 × 120 cm, which you can change under "Door and window sizes".
4. Add chimney breasts and alcoves. Add the depth of each side of a chimney breast to the room's width, twice: a breast 40 cm deep adds 80 cm of wall.
5. Read the tin. Check its coverage and how many coats it suggests before you buy.

Mistakes that leave you short, or with spare tins

• Using the floor area. A 4 × 3 m room has 12 m² of floor but about 31 m² of wall. Paint the walls by the floor area and you run out at the second wall.
• Forgetting the second coat. One coat at 13 m² a litre is half the paint you need.
• Buying two small tins. Two 2.5 l tins cost £14 more than one 5 l tin at B&Q.
• Trusting "up to". The tin's figure is for a smooth, sealed wall. Rough, porous or patched walls take more.
• Skipping the mist coat. On new plaster, full-strength emulsion can peel. Budget one more coat.
• Choosing the colour in the shop. A colour chosen under shop lights can look different at home. Try a tester on your own walls first.

Living with magnolia you cannot change? Magnolia walls: what goes with them, and how to ask to paint covers the colours, woods and bulbs that work with it. Wallpapering instead? The wallpaper calculator counts the rolls.

See the colour in your own room

Snuug, the iPhone room app, rebuilds your room in 3D from one slow walk round it. Paint your walls in Snuug and see the colour in your own room, by your own windows and next to your own sofa, before you buy a tin. Your room comes to within about 3% of its real size in our tests of ten homes, so you can try a feature wall, a full room or a darker ceiling, and buy only the tin you choose.

Questions

How much paint do I need for a 3 × 4 m room?

With 2.4 m ceilings, one door and one window, the walls come to about 30.7 m². Two coats at 13 m² per litre is 4.7 litres: one 5 l tin, or a 5 l and a 2.5 l if you want B&Q's 10% spare. The ceiling (12 m²) needs another 1.8 litres: one 2.5 l tin.

How much does a 5 litre tin of paint cover?

At 13 m² per litre, a 5 l tin covers about 65 m² with one coat, or 32 m² with two. That is the walls of a 4 × 3 m room with 2.4 m ceilings, one door and one window. On rough or porous walls it covers less.

How much paint do I need for a ceiling?

Multiply the room's length by its width, then by two coats, and divide by the coverage on the tin. A 4 × 3 m ceiling is 12 m², so it needs about 1.8 litres of paint at 13 m² per litre: one 2.5 l tin. Some ceiling paints cover less per litre than wall paint, so check the tin.

Do I need two coats of paint?

Usually, yes. Homebase recommends two coats for most interior surfaces, and Dulux says two coats for its EasyCare Washable & Tough. Dulux also sells one-coat paints (Simply Refresh). A big change of colour, dark to light, can need a third coat.

How much paint do I need for a mist coat?

No paint maker gives a figure, so the calculator counts the mist coat as one more full coat: the water stretches the paint, and the dry plaster soaks it up. For the walls of a 3 × 3 m room that is about 2 litres of emulsion before you add the water. B&Q says to thin it two parts emulsion to one part water.

Is it cheaper to buy 5 litre tins of paint?

Usually. At B&Q on 31 October 2026, Dulux Walls & Ceilings matt cost £36 for 5 litres (£7.20 a litre) and £25 for 2.5 litres (£10 a litre). Two 2.5 l tins cost £50, so one 5 l tin is cheaper whenever you need more than 2.5 litres.

Editorial notes

Written by the Snuug team and checked on 1 November 2026. A number with no source beside it is a designers' rule of thumb, not a study.

1. B&Q, Dulux Walls & Ceilings matt emulsion 2.5 l (up to 13 m² per litre on average; £25), read 31 Oct 2026
2. B&Q, Dulux Walls & Ceilings matt emulsion 5 l (£36, £7.20 a litre; 70 m² a tin in the specification), read 31 Oct 2026
3. Screwfix, Dulux Pure Brilliant White silk emulsion 2.5 l (13 m² per litre), search summary 31 Oct 2026
4. B&Q, Wall painting calculator (10 m² per litre; at least 10% extra), read 31 Oct 2026
5. Homebase, Paint calculator (10 m² per litre; two coats; optional 10% buffer), read 31 Oct 2026
6. Dulux UK, How much paint do you need? (the method; rough tin sizes by room), read 31 Oct 2026
7. B&Q, How to paint new plaster (mist coat: two parts emulsion, one part water), read 31 Oct 2026
8. Howarth Timber (Green, 14 Sep 2026), UK door sizes (762 × 1981 mm), checked 31 Oct 2026

Use this from an AI assistant

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