Paint calculator: how much paint do I need, in litres per m²

Paint needed is worked out as litres = area in m² × number of coats ÷ coverage in m² per litre. At 13 m²/L and two coats, each square metre takes 0.15 L, so a 10 m² wall needs 1.54 L.

The formula: area × coats ÷ coverage

Coverage, sometimes called the spreading rate, is the area one litre of paint covers in a single coat, in square metres per litre. It is printed on the tin and on the technical data sheet. The sum is then straightforward: litres = area × number of coats ÷ coverage.

Example: a wall 4 m wide and 2.50 m high is 10 m². Two coats of a matt emulsion covering 13 m²/L need 10 × 2 ÷ 13 = 1.54 L, or about 0.15 L per square metre.

Litres per m² and area covered by each tin

The table applies the formula to two UK figures: 13 m² per litre per coat for Dulux Walls & Ceilings matt emulsion, and 17 m² per litre on smooth surfaces for Crown Trade Matt Vinyl. Example: two coats at 13 m²/L need 0.154 L for 1 m², and a 5 L tin covers 32.5 m².

What the coverage figure means

B&Q lists Dulux Walls & Ceilings matt emulsion as covering up to 13 m² per litre on average. Trade paints are often quoted higher: the Crown Trade Matt Vinyl data sheet gives 17 m² per litre, and it says on smooth surfaces. Rough plaster, textured walls and porous surfaces take more paint than a smooth, previously painted wall.

The calculator starts at 13 m²/L, the more cautious of these figures. If your tin gives a different coverage, type it in: the method stays the same.

Coats, mist coats and new plaster

Crown Trade advises two coats of Matt Vinyl as required, noting that certain colours need extra coats for full opacity. On porous surfaces such as bare plaster, the same sheet calls for a mist coat first: Matt Vinyl diluted with 40% clean water by volume. Because coverage is quoted per coat, each coat adds the same amount: 30 m² takes 2.31 L in one coat, 4.62 L in two and 6.92 L in three at 13 m²/L.

Tick the primer box to count a mist coat or a primer separately, with the coverage printed on that product.

How much paint for a room

Measure the walls, not the floor: wall area is the perimeter of the room times the ceiling height. A 3 × 3 m room with a 2.4 m ceiling has 12 m of wall and 28.8 m² to paint; two coats at 13 m²/L need 4.43 L, so one 5 L tin. A 4 × 3 m bedroom of the same height has 33.6 m² of wall and needs 5.17 L. The ceiling is a separate area: 12 m² more for that bedroom, or 1.85 L in two coats.

Take off large windows and doors before you enter the area, or the result will lean towards paint left over.

Which tins to buy

Matt emulsion is commonly sold in 2.5 L and 5 L tins, which are the calculator's starting sizes; the Crown Trade range runs from 250 ml to 10 litres. The calculator looks for the mix of tins that reaches the volume with the least left over, then with the fewest tins. For 4.43 L, one 5 L tin. For 5.17 L, one 5 L tin and one 2.5 L tin. Add other sizes in the box, for example "10 ; 5 ; 2.5", if your merchant stocks them.

Crown advises intermixing packs for colour consistency when a job takes several tins of the same colour.

Common mistakes

• Forgetting the second coat: 30 m² in two coats is 60 m² of paint to spread.
• Using floor area instead of wall area: the 4 × 3 m bedroom has 12 m² of floor but 33.6 m² of wall.
• Taking the best-case coverage from a trade data sheet for new plaster.
• Leaving out the mist coat on bare plaster.

To measure the room, use the wall area of a room; wallpaper rolls are counted in drops rather than litres, and the mitre angle helps when you fit new skirting or coving before painting.