The two are limited by different things

Cabin luggage is limited by size. There is a physical box — a sizer at the gate, or the space under the seat in front — and your bag either fits or it does not. Weight limits exist, usually 10kg, but size is what gets bags refused.

Checked luggage is limited by weight. Hold size limits are generous enough that almost any suitcase clears them: British Airways allows 90 × 75 × 43cm, and easyJet works to a 275cm total of length plus width plus height. What costs money is the scale at the desk.

Understanding that difference is most of the decision.

When cabin-only makes sense

Up to about a week, for one person who packs sensibly. You save the hold fee, you skip the baggage carousel, and your bag cannot be mislaid. On low-cost carriers the saving is real — often more than the difference in the price of the case.

The catch is the 10kg cabin limit on most carriers. A cabin case weighing 2.5kg empty leaves you 7.5kg, which is enough but not generous.

When to check a bag

Longer than a week, travelling with others, or carrying anything bulky — walking boots, a coat, gifts, sports kit. Also any trip where you would rather not think about it.

Once you are checking a bag, buy the size the trip needs rather than the smallest that qualifies. The fee is the same for a medium or an extra large.

Empty weight matters more than people think

On a 23kg checked allowance, a case weighing 3.4kg empty leaves 19.6kg for contents. One weighing 5.3kg leaves 17.7kg. That is nearly two kilos of difference before you have packed a thing — roughly a pair of shoes and a coat.

It is the single most overlooked number when people buy checked luggage, which is why we list it on every product page.

The middle path

A cabin case in the locker and a small underseat bag covers a surprising amount of travel. On carriers that include both bags free — British Airways, Jet2, TUI — that combination is often all you need for a week.