The depth distribution: why 15 cm is a wall and 20 cm is a crowd

Short answer: of the airlines that publish a personal-item size, the depth values cluster hard: seven sit at 15 cm, seven more at 20 cm (or 20.3 cm), and the rest trail at 21.6, 22.9 and 25 cm. There is no carrier between 15 and 19 cm. A 14 cm-deep bag sits alone on the safe side of the only gap that matters.

The numbers, binned

We took every published personal-item depth and binned it. The result is not a spread — it is two stacks with a cliff between them.

Depth	Carriers	Count
15 cm	Lufthansa, SWISS, Air France, KLM, British Airways, Iberia, Condor	7
20 cm	Ryanair, easyJet, Transavia, Vueling, Wizz Air	5
20.3 cm	American, Spirit	2
21.6 cm	Southwest	1
22.9 cm	United	1
25 cm	Eurowings	1

Condor is an addition to the original 16-carrier depth study, which is why the published count here is seventeen rather than sixteen; it uses the same 40 × 30 × 15 cm box as the seven legacy carriers.

The gap that decides trips

Nothing publishes a depth between 15 and 19 cm. That empty band is the wall: a bag at 14 cm clears seven carriers, and a bag at 16 cm loses all seven at once. The crowd at 20 cm is where most low-cost and US carriers live, so 20 cm is "safe almost everywhere" — but the seven at 15 cm are the ones with the strict boarding policies and the gate fees.

What 14 cm buys

Our bag is 14 cm deep — one centimetre inside the wall, and on the safe side of the only gap in the entire distribution. That single centimetre is the difference between clearing seven carriers and losing them.

Should you buy the Jelkin 25 for this?

No carrier publishes a depth between 15 and 19 cm. That empty band is the wall, and 14 cm sits on the safe side of it — one centimetre that decides seven carriers at once. If the bag you are considering is 16 cm deep, this is not the right comparison; if it is 14 cm, you have bought the entire margin available. That single centimetre is what you are paying for.

How to check this yourself before you fly

1. Measure depth only. Length and width are almost never the failing axis on a personal item.
2. Compare against 15 cm, not 20 cm. The 20 cm crowd is real but the 15 cm group is where gate fees live.
3. Measure packed. A bag that is 14 cm empty can be 16 cm full, and 16 cm loses all seven carriers.
4. Keep a tape measure in the bag. Being able to demonstrate 14 cm at the gate ends most arguments.

Keep reading:

• airline personal item size checker — all 19 carriers side by side
• one-centimetre depth cliff — why 15 cm decides six airlines
• the 25 cm bag that fits every sizer — length-vs-depth end to end
• why external dimensions lie about capacity — how we measure bags
• Jelkin 25 — the 25 × 17 × 14 cm bag measured on this page