One or two nights — underseat

Around 20 to 24 litres. An underseat case at 40 × 30 × 20cm holds a change of clothes, washbag and a laptop. Travels free on almost every carrier, which makes it the cheapest way to take an overnight trip.

A long weekend — small cabin

Around 30 to 35 litres. Three or four days of clothes with room for a pair of shoes. On easyJet this size travels free; elsewhere it is either the free bag or a light Priority case.

About a week — full cabin suitcase

Around 45 to 57 litres. This is the sweet spot for cabin-only travel: seven days if you pack an outfit a day and wear one twice. You avoid the hold fee entirely, and the bag stays with you.

The limit here is weight rather than space. Most carriers allow 10kg including the case.

One to two weeks — medium checked

Around 65 to 83 litres. Ten days for one person, or a long weekend for two sharing. Once you are checking a bag, this is the size most people actually need — and the empty weight of the case starts to matter against a 23kg allowance.

Two to three weeks — large checked

Around 105 to 134 litres. A fortnight for one, or a week for two sharing one case. Watch the scales rather than the space: at this size you can pack past 23kg without filling it.

A month or more, or family travel — extra large

130 litres and up. Relocations, cruises, long-haul and family holidays where several people share. Almost always weight-limited well before it is full, so pack to the scales.

Two adjustments

Sharing: two people in one case need about 1.5 times what one needs, not double. A family sharing needs roughly twice one person’s space.

Style: take 20% off if you genuinely wear things twice; add 30% if you pack shoes, coats and just-in-case items.