Most bags that fail at a gate were measured at home and passed. The measuring was the problem, not the bag.
A gate sizer does not read your label, care what the listing said, or make allowance for a strap. It is a metal box. Either the packed object drops into it or it does not.
1. Pack it the way you will actually fly
An empty bag measures smaller and tells you nothing. Soft bags spread when they are full, and the panel that was flat in the shop bulges once there is a jumper behind it.
Pack it for the trip, then measure. If it only passes empty, it does not pass.
2. Flatten the straps against the back panel
Shoulder straps, a sternum strap and a waist belt all stand proud of the back. On a bag where the entire depth allowance is twenty centimetres, padded straps standing a centimetre off the panel have taken five per cent of it before you have packed anything.
Push them flat. A sizer makes no allowance for a strap, and neither does the person watching you use it.
3. Measure the widest point of every face
Not the fabric panel. The whole object, including handles, zip pulls, buckles, feet and any pocket that is currently full.
easyJet say this explicitly: their allowance is measured including handles and wheels. Assume every other carrier means the same thing, because a metal frame has no way of ignoring a wheel.
4. Compare against the airline you are flying
Not the one you flew last time. The allowances differ, and they change.
Four of the six UK carriers we track set 40 x 30 x 20. easyJet allow 45 x 36 x 20. British Airways are tighter. Every figure, with the date we last checked it, is in the under-seat cabin bags guide.
The trick worth knowing
A sizer is a box, so a bag can be turned. What matters is your three measurements against the frame's three, largest against largest, middle against middle, smallest against smallest.
That is why a bag built to Ryanair's old 40 x 20 x 25 still goes into a current 40 x 30 x 20 frame, turned on its side. Sorted, it is 40, 25, 20 against 40, 30, 20, and every number clears. We wrote that one up in full: 40x20x25 or 40x30x20.
What people get wrong
- Trusting litres. Capacity tells you nothing about whether a bag goes into a frame. Two bags of the same volume can be different shapes and only one fits.
- Measuring internal dimensions. The gate measures the outside.
- Measuring it empty. See step one. This is the common one.
- Assuming a label is a measurement. Plenty of bags sold as 40 x 30 x 20 are that size across the fabric panel and larger across the object.
- Believing the word approved. No airline runs an approval scheme for luggage.
If it does not fit at the gate
It goes in the hold and you pay the gate rate, which is the most expensive way to put a bag on a plane. That is the whole reason to spend five minutes on this at home.