Cabin Backpacks

Wheels and a telescopic handle are structure, and structure takes up allowance from the inside. In a box of 40 x 30 x 20 that is a meaningful share of the total, and none of it carries anything.

A backpack spends almost all of its external size on the inside. It also weighs less before you have put anything in it, which counts on the two carriers that set a weight.

The trade is that you carry it rather than wheel it. For a bag this size, across an airport, that is not much of a trade.

This is the real difference between a backpack that passes a sizer and one that does not.

Shoulder straps, a sternum strap and a waist belt all sit proud of the back panel. On a bag where the whole depth allowance is twenty centimetres, straps that stand a centimetre off the panel have taken five per cent of it before you pack anything.

Padding makes it worse, because the padding is thickest exactly where the strap meets the panel. So look for straps that lie flat against the back, and flatten them yourself before you measure anything at home.

Then measure the whole object. The widest point of each face, not the fabric panel.

Dimensions before litres. A capacity figure tells you nothing about whether a bag will go into a frame, and a listing that leads with capacity is usually avoiding the other number.

Check whether the measurement is external or internal. Internal dimensions on a bag this size are close to meaningless, because the thing being measured at the gate is the outside.

Look for a square base. Tapered bags look sleeker in photographs and lose you the corners of the allowance.

And be wary of the word approved. No airline runs an approval scheme for luggage. A bag either measures inside the published allowance or it does not.