Paint Quantity — Method
Paint quantity is an area calculation adjusted for coats and the products spreading rate. Litres = (net surface area x number of coats) / coverage, where coverage (m2 per litre) comes from the tin - typically 10 to 12 m2/L for matt emulsion, less for textured surfaces. Subtract large openings such as doors and windows. New or porous surfaces absorb more on the first coat, so add a primer or mist coat, or simply allow extra paint for that coat.
Frequently Asked Questions
Litres = (wall area x coats) / coverage. For 40 m2 of wall, 2 coats, at 11 m2 per litre that is about 7.3 litres. Deduct doors and windows from the area first.
The area one litre covers in a single coat, printed on the tin - typically 10-12 m2/L for matt emulsion, lower for textured, deep-colour or exterior paints.
Multiply each wall length by its height and add them up, then subtract the area of doors and windows. For a whole room, add the four walls (and ceiling if painting it).
Usually two top coats for an even finish, plus a primer/undercoat on bare or patchy surfaces or when changing colour drastically. Count the primer as an extra coat in the calculation.
New plaster and porous surfaces absorb the first coat, lowering the effective coverage. Use a diluted mist coat or a primer, or add roughly 10-20% extra paint for that first coat.
Estimating Paint Quantities
Working out paint is a matter of area, coats and the products spreading rate. Get those three right and you avoid both running short mid-wall and over-buying tins you will not use.
The method
Net area (walls minus openings) x number of coats / coverage in m2 per litre. Round up to whole tins for ordering.
Surfaces and coats
Most jobs need two top coats; bare or strongly coloured surfaces need a primer or extra first coat because they absorb more. Always check the coverage figure on the specific product, as it varies a lot between matt, silk, exterior and textured paints. For wall quantities of other materials see the brick and concrete calculators.
Related: Brick, Concrete Mix.