The low-cost personal-item trap: the free box is small, the paid one is not

Short answer: on Ryanair, Wizz Air, easyJet, Transavia, Vueling and Spirit the only thing that is free is the small under-seat box — and on five of them that box is exactly 40 × 30 × 20 cm. A 25 × 17 × 14 cm bag fits it with room to spare. The trap is the second, larger "cabin bag," which is where the fees live.

Free means one small box

Legacy carriers bundle a carry-on and a personal item into the fare. Low-cost carriers unbundle them. What stays free is the under-seat personal item, and on most of Europe's low-cost carriers it is the same size.

Why the trap catches people

The free box is generous in volume (24 L on five carriers) but strict in depth: 20 cm. A soft bag packed to 21 cm deep fails the sizer even though it is "within the volume." The paid step-up exists precisely because travellers underestimate how fast a soft bag gains depth. Our bag is 14 cm deep empty and stays under 20 cm with normal packing, so it clears the free box and you never need the paid step.

The honest limit of this advice

Low-cost sizers are physical boxes at the gate. A bag that is within the published numbers on paper can still be pulled if the gate agent judges it overstuffed. The margin that matters is depth, and 14 cm against a 20 cm cap is a comfortable six centimetres — until you force the front pocket shut.

Should you buy the Jelkin 25 for this?

On Ryanair, Wizz, easyJet, Transavia and Vueling the only free thing is a 40 × 30 × 20 cm under-seat box, and the paid step-up is where the fees live. The Jelkin 25 is 14 cm deep against that 20 cm cap — six centimetres of margin, which is the practical difference between never buying priority and buying it on every flight. If you fly low-cost more than twice a year, the bag pays for itself in avoided add-ons.

How to check this yourself before you fly

1. Find the free under-seat box for your exact carrier. Five European low-cost carriers use the identical 40 × 30 × 20 cm.
2. Measure depth, not volume. 24 L of volume is generous; 20 cm of depth is the constraint.
3. Do not force the front pocket shut. Overfilling the pocket is the single most common way a compliant bag gains the centimetre that fails it.
4. Price the add-on before you fly. Knowing a cabin bag costs €40–60 makes the margin on a 14 cm bag concrete.

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