Work out how much paint a room actually needs, and which tin sizes to buy, from its dimensions. Coverage varies by paint and wall surface — check the figure on your own tin and adjust it below if it differs from the default.
Typical UK internal door, frame included. Adjust if yours differs.
Typical mid-size window. Adjust if yours differs.
12 m²/L is a common conservative default. Check your tin — smooth walls can reach 16-17, bare plaster can be well under 12.