How much paint do I need for a room?
A standard 4m × 3m room with a 2.4m ceiling has about 31m² of wall once you take off a door and a window. At 13m² per litre and two coats, that is around 4.8 litres — one 5L tin.
Coverage rates
Most UK emulsions claim 12–14m² per litre per coat on a smooth, sealed surface. Matt emulsion goes further than silk. Textured, porous or dark-to-light surfaces all reduce coverage, sometimes by a third.
Always plan for two coats. One coat only works when you are repainting the same colour on a sound surface.
New plaster needs a mist coat
Fresh plaster is thirsty and will suck a normal topcoat straight in, leaving a patchy finish that peels. Seal it with a mist coat first: matt emulsion thinned with about 20% water, applied as a full extra coat. Budget for that litre — most people forget it entirely.
Tin sizes and cost
UK tins come in 2.5L and 5L. A 5L tin is usually cheaper per litre than two 2.5L tins, so when you need 4.6 litres buy the 5L. When you need 6 litres, a 5L plus a 2.5L is normally better value than two 5L tins — and you keep a part tin for touch-ups.
Woodwork and ceilings
Work these out separately. Ceilings take the same emulsion but a whiter shade; woodwork uses gloss, satin or eggshell at roughly 16m² per litre. A typical door takes about 0.2 litres per coat on each side, and skirting roughly 0.1 litres per metre for two coats.