Paint Calculator

Litres = area in m² × coats ÷ coverage per litre. The calculator uses 10 m² per litre, a realistic figure for emulsion rolled onto ordinary walls. Tins often claim 12–14 m², but that assumes a perfect surface and a thin, even coat.

Add the lengths of all the walls to get the perimeter, then multiply by the ceiling height. A 4m × 3.5m room with a 2.4m ceiling has a 15m perimeter and 36 m² of wall. You can subtract doors and windows, but leaving them in gives a useful margin for touch-ups.

Two coats is standard when repainting in a similar colour. Plan for three when going from dark to light, or when using strong reds and yellows, which cover poorly. Adding a coat raises the paint needed by half — 36 m² goes from 7.2 to 10.8 litres.

Use a primer or mist coat on new plaster, new plasterboard, bare wood, and over stains or very dark colours. Fresh plaster absorbs paint, so skipping it often means an extra coat of the more expensive topcoat.

Often, for coverage rather than finish. If a premium paint covers in two coats where a budget one needs three, the cost difference shrinks and you save a day of work. It also tends to be more washable, which matters in hallways and kitchens.

Paint is sold in 2.5L and 5L tins, and 5L is usually cheaper per litre. Buy enough for the whole job at once — tins from different batches can differ slightly in colour, which shows on a large wall.

A little left over is useful for touch-ups. Store it sealed and upright, and write the room and date on the lid.

Measure the floor: a ceiling has the same area. The 4m × 3.5m bedroom has a 14 m² ceiling, needing 2.8 litres for two coats. That is just over one 2.5L tin, so buy a second small tin or a 5L tin of ceiling white.

Woodwork uses gloss, satin or eggshell rather than emulsion, and is usually sold in 750ml and 2.5L tins. It covers less area per litre, but the areas are small — one 750ml tin usually does the skirting and door frames of a typical room.

Masonry paint on rough render or pebbledash can cover half the area per litre that emulsion does on smooth plaster. Double the litres for rough surfaces, and check the coverage printed on the tin before buying.

Fill cracks and holes, sand the filler smooth, and wash marks off before painting. Paint does not hide an uneven surface — it highlights it, especially in sheen finishes.

Most water-based emulsions are touch dry in an hour or two but need longer before recoating. Rushing the second coat can lift the first. Follow the tin, and allow more time in a cold or damp room.