Enter your wall area (or room size), the number of coats and any doors and windows — get the litres and cans to buy.
A paint calculator tells you how much paint you need so you buy the right amount in one trip. The method: take the wall area, subtract the openings (doors ≈ 1.8 m², windows ≈ 1.4 m² each), multiply by the number of coats, then divide by the paint's coverage — typically 10–12 m² per litre for emulsion. This tool then rounds up to whole cans at your chosen can size.
Use two coats for a solid, even colour — three when going light over dark or covering stains. Porous or fresh plaster drinks more paint, so add 10–15% and consider a primer/mist coat first. Buying all your paint from the same batch avoids slight colour differences between cans.
Related: Tile Calculator · Unit Price Calculator