Carry-on dimensions for international flights

IATA has published suggested cabin bag guidance in the past, but it is advisory and carriers are free to ignore it. Most do, because cabin space is a commercial asset: an airline with smaller overhead bins has a reason to publish a smaller limit, and one selling priority boarding has a reason to publish a free bag small enough to make the upgrade attractive.

The practical result is a narrow band rather than a standard. Dimensions cluster around 55 x 40 x 23 cm, which is within a centimetre or two of the US 22 x 14 x 9 inch norm, and the real variation is in weight and in how many bags you get free.

A US domestic traveler can fly for years without a cabin bag ever being weighed. Seven kilograms is 15.4 lb - and an empty hardside carry-on can be a third of that before a single shirt goes in. This is the single biggest reason a bag that has always worked at home fails abroad.

• Buy for empty weight if you fly weighed carriers. It is the most important figure on the spec sheet, ahead of capacity and ahead of features.
• Weigh the packed bag at home. Not at the airport, where the only available answer is a fee.
• Remember the personal item usually has its own limit abroad. Moving weight into a second bag is not automatically a solution.
• Expect enforcement to vary by airport, not by rule. The rule is published either way, which means a bad day at the gate is yours to absorb.

On easyJet and Wizz Air the free allowance is a small bag that must fit under the seat - 45 x 36 x 20 cm and 40 x 30 x 20 cm respectively on the pages we checked. The familiar cabin trolley exists, but it is bundled into a paid product: easyJet's large cabin bag at 56 x 45 x 25 cm and 15 kg, Wizz Air's 55 x 40 x 23 cm trolley at 10 kg with WIZZ Priority.

That changes what you should buy. For these carriers the valuable bag is a well-organized under-seat bag, not a bigger suitcase - see best personal item bags and best underseat carry-on bags. US ultra-low-cost carriers run the same model with different numbers, which personal item size by airline covers.

1. Pack the bag as you will actually travel with it, including the expansion zip if you intend to use it.
2. Measure at the widest points: over the wheels, over the handle housing, over any external pocket that is full.
3. Convert once and write both numbers on a note in your phone. 55 x 40 x 23 cm is 21.7 x 15.7 x 9.1 inches.
4. Weigh it, in kilograms, because that is the unit the limit is published in.

A retailer describing a bag as cabin size is making a marketing claim, not a measurement. Cabin baggage size goes into how the measurement is taken and what to do when you are a centimetre over.

Close to it, if you buy to the strictest limit you fly regularly rather than the most generous. A bag inside roughly 55 x 38 x 22 cm clears the Emirates figure, the common 55 x 40 x 23 figure, and the US 22 x 14 x 9 standard - at which point your remaining problem is weight rather than shape.

What does not travel well is a bag bought to a generous domestic limit. A 24 x 16 x 10 inch case built for Southwest or Frontier is oversized almost everywhere else in the world. Best luggage for international travel is the shortlist built on exactly this constraint.