This is a guide. Surface texture, primer condition, and brand all affect coverage — always check your tin.
This calculator works out the total wall area of a room, deducts a standard allowance for doors and windows, then divides by your chosen coverage rate and multiplies by the number of coats. Coverage rates vary between paint types and brands — the slider lets you match whatever your tin specifies. The result is rounded up to the nearest half-litre (or tenth of a gallon) so you don't run short.