Underseat means under the seat in front

An underseat bag is one that fits in the footwell space in front of your seat, rather than in the overhead locker. On most low-cost carriers this is the free allowance, which is why it matters commercially: it is the bag you do not pay for.

Depth is the constraint

People focus on height, but the footwell is shallow. A case advertised as fitting a 40 × 30 × 20cm allowance must be no more than 20cm deep including its wheels. Wheels typically add 2 to 3cm, so a case with an internal depth of 20cm will not pass.

Buy to the external measurement. It is the only one the sizer cares about.

The two common box sizes

Most UK carriers use one of two underseat allowances:

• 40 × 30 × 20cm — Ryanair, Wizz Air, Jet2 and TUI. The most common size, and the safest to buy for if you fly several carriers.
• 45 × 36 × 20cm — easyJet. More generous, and a case bought to this size will not fit the 40cm carriers.

British Airways is the outlier at 40 × 30 × 15cm, which is too shallow for any wheeled case. Their full cabin bag is included free, so buy for that instead.

Wheeled or not?

Wheels cost you depth and about 400g, but they matter across an airport. For a case at the 40cm size, wheels are usually worth it. Below that, a soft holdall or backpack packs more into the same box because there is no wheel housing.

What actually fits

A 40 × 30 × 20cm case holds around 20 to 24 litres — two or three days of clothes, or a laptop and an overnight change. It is a genuine short-trip bag, not just a personal item.