Carry-on size guide

There is no international standard for carry-on size. Each airline sets its own, and the convergence around 22 x 14 x 9 inches in the United States is a convention rather than a rule. Outside the US, and on budget carriers everywhere, the allowance is frequently smaller and more aggressively enforced.

Two numbers matter per airline. The carry-on allowance is what goes in the overhead bin. The personal item allowance is what goes under the seat in front of you, and on basic economy fares it is often the only thing included.

Crucially, the stated dimension includes wheels, handles and any external pocket when it is full. A bag advertised as 22 inches is often 23 inches with its wheels, which is exactly the inch that fails a sizer.

The metal frame at the gate is the real rule, and it is unforgiving in a specific way: it is a rigid box, and your bag has to drop in without being pushed. A soft bag that is slightly over will often go in with a shove, and a hard-shell one that is slightly over will not go in at all.

Enforcement is inconsistent, which is what makes this stressful. On a full flight, on a budget carrier, or on a route known for overhead congestion, gate agents check. On a half-empty midweek domestic flight they usually do not. Planning around the lenient case is how people end up gate-checking a bag they needed on board.

The consequence of failing varies from free gate-checking to a fee considerably larger than the cost of checking the bag in the first place. On several budget carriers the gate fee is deliberately punitive, and that is the scenario worth packing to avoid.

A hard-shell suitcase is a fixed volume. It either fits or it does not, and if the airline's allowance is smaller than the case, there is nothing you can do at the gate.

A soft-sided bag or a travel backpack compresses. It can be over-packed into a shape that fails, and it can equally be squeezed, re-packed or partly emptied into a jacket to pass. That flexibility is the single strongest argument for soft luggage on budget and international carriers.

The cost of soft is protection and structure. A hard shell protects fragile contents and stands up on its own; a soft bag slumps. Most travelers end up choosing by route rather than by preference -- hard shell for US domestic, soft for anything where the allowance is tight. The broader comparison is in backpack vs suitcase.

Manufacturer dimensions are frequently the dimensions of the shell, not the bag. Measure it yourself, packed, and you will know rather than hope.

Most people fail a sizer by depth, not length, and depth is the dimension that packing technique controls.

Compression helps and has a trap in it. Compression cubes reduce the volume of clothing but they do not reduce weight, and a fully compressed bag that is rigid cannot be squeezed at the gate. The tradeoffs are in packing cubes vs compression bags.

Wear the bulk. A jacket, boots and a sweater worn through the airport are items with no size or weight cost. This is the single most effective tactic on a tight allowance and the one budget-carrier regulars rely on. More of the method is in how to pack light.

Keep the personal item genuinely under-seat sized. A personal item that is too big for the footwell is the second most common failure, and on a basic economy fare it is the only bag you have.