Room dimensions and door/window count in — exact litres needed and the best combination of tins to buy, out.
Standard UK emulsion covers 12-14 m² per litre on a smooth, primed surface — this calculator defaults to 13 m²/litre as a reasonable mid-point. But textured, porous, or previously unpainted (bare plaster) walls can eat up to 30% more paint on the first coat, and darker colour changes sometimes need an extra coat to fully hide what's underneath. Check your specific tin's stated coverage and adjust the rate above if it differs.
A single coat rarely gives even, opaque coverage — brush and roller marks, patchiness, and the previous colour showing through are all common with just one. Two coats is the standard recommendation for a professional-looking finish on any colour change.
A 4m × 3.5m room with 2.4m walls, one door and two windows: wall area is 36 m², minus 4.4 m² for openings, leaving 31.6 m² paintable. Two coats means 63.2 m² of total coverage needed, which at 13 m²/litre works out to 4.86 litres — buy two 2.5L tins (5L total).