Here's the maths, and it's dead simple once you've done it once.
Step one: work out your wall area. Measure the perimeter of the room (add up all four wall lengths) and multiply by the ceiling height. A 4m x 4m room with 2.4m ceilings gives you (16 x 2.4) = 38.4m². You can deduct big openings — a standard door is about 1.8m², a decent window 1.5m² — but honestly, don't bother subtracting the small stuff. The extra paint is your safety margin.
Step two: divide by coverage. Most emulsions cover 11–13m² per litre per coat. F&B Estate Emulsion and Modern Emulsion sit around there; Little Greene Intelligent Matt is similar. So 38.4m² ÷ 12 = 3.2 litres per coat.
Step three: multiply by coats. Almost always two. That's 6.4 litres, so you'd buy two 2.5L tins plus a 1L, or round up to a 5L if the maths is close.
The big variable is colour change and the surface. Going dark over light — say Hague Blue or Studio Green over a pale wall — you may need three coats, or a tinted basecoat first. Strong reds and yellows are notoriously poor coverers; budget extra. Fresh plaster needs a mist coat (watered-down emulsion) which drinks paint, so add a litre or two.
A few real-world pointers:
- Ceilings at 4x4m = 16m², so roughly 1.5L per coat in a flat white like Wevet or Strong White.
- Always buy from the same batch where you can, and keep a noggin of leftover for touch-ups — colours drift slightly between batches.
- Trim uses far less than you think; a 750ml tin of eggshell does the woodwork in most rooms over two coats.
When in doubt, round up. Running out three-quarters through a wall and waiting on a fresh tin is the fastest way to a visible lap mark. Better a spare tin in the cupboard than a patchy finish.