Start with the trip, not the case

The question is rarely “what size suitcase” in the abstract. It is how many nights, whether anyone is sharing, and how you pack. Get those three right and the size follows.

Nights

As a working guide, one person packing normally needs roughly:

• A night or two — around 20 to 25 litres. An underseat case.
• A long weekend — around 30 to 35 litres. A small cabin case.
• About a week — around 45 litres. A full cabin suitcase, if you are disciplined.
• One to two weeks — around 70 litres. A medium checked case.
• Two to three weeks — around 100 litres. A large case.
• A month or more — 130 litres and up. Extra large, or two cases.

Sharing

Two people sharing one case need roughly 1.5 times what one person needs — not double. Shared items like toiletries, chargers and a hair dryer only travel once. A family sharing needs around twice one person’s allowance for the same reason.

This is why one large case often beats two mediums: fewer bags to move, and less duplicated packing.

How you pack

If you genuinely wear things twice, take 20% off the figures above. If you pack shoes, coats and just-in-case items, add 30%. Be honest here — aspiration is how people end up repacking on the airport floor.

Then check the allowance

Once you know the litres, check it against your airline. If the answer is cabin-sized, buy to the tighter of the two allowances your carrier offers. If it is hold-sized, the constraint changes from size to weight, and the empty weight of the case starts to matter.

When in doubt, size down

A case you can fill is better than one you cannot lift. Weight limits catch more people than size limits, particularly on checked bags where 23kg arrives faster than most expect.