The US personal-item standard is 18 × 14 × 8 inches — and it is bigger than you think

Short answer: American and Spirit publish 18 × 14 × 8 inches (about 46 × 36 × 20 cm). Southwest allows 18.5 × 8.5 × 13.5 in and United 9 × 10 × 17 in. Converted, all four clear a 25 × 17 × 14 cm bag on every axis — the US limit is actually more generous in volume than Europe's 40 × 30 × 15 cm wall.

Three numbers, two shapes

The US "personal item" is usually quoted in inches, and the order of the numbers is not always length-width-depth. We converted each to centimetres and identified the strict axis.

Carrier	As published	In cm (L × W × D)	Strict axis
American	18 × 14 × 8 in	45.7 × 35.6 × 20.3 cm	depth 20.3 cm
Spirit	18 × 14 × 8 in	45.7 × 35.6 × 20.3 cm	depth 20.3 cm
Southwest	18.5 × 8.5 × 13.5 in	47.0 × 21.6 × 34.3 cm	width 21.6 cm
United	9 × 10 × 17 in	22.9 × 25.4 × 43.2 cm	width 25.4 cm

Bigger than the European wall

Europe's strictest carriers cap the personal item at 40 × 30 × 15 cm. American and Spirit's 20.3 cm depth is looser, and their length/width are wider. A 25 × 17 × 14 cm bag is 17 cm wide and 14 cm deep — inside every US limit. The one to watch is United's 25.4 cm width: a 30 cm-wide tote would fail there even though it is shallow.

Why travellers still get stopped in the US

US gate agents enforce the under-seat test more than the printed box. A bag that is technically within 18 × 14 × 8 in but bulging past it visually gets flagged. The fix is the same as in Europe: keep depth honest. At 14 cm our bag sits six centimetres inside American/Spirit's depth and comfortably inside United's width.

Should you buy the Jelkin 25 for this?

The US 18 × 14 × 8 in standard converts to 45.7 × 35.6 × 20.3 cm — looser in depth than Europe's 15 cm wall, but United's 25.4 cm width is the tightest width in our whole set. The Jelkin 25 is 17 cm wide and 14 cm deep, so it clears United's width with 8 cm to spare and every US depth with more. Choose it over a wide tote if you fly United or connect domestically.

How to check this yourself before you fly

1. Convert the published inches to centimetres before you compare. American and Spirit publish 18 × 14 × 8 in; Southwest and United publish different orders entirely.
2. Identify the strict axis — for United it is width, not depth. Most people only ever check depth.
3. Measure the bag at its opening when full. A soft tote is widest where it opens, not at the base.
4. If you fly both US and European carriers, size to 15 cm depth. It clears both regimes and removes the mental arithmetic.

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