The formula
linear sum = length + width + height
Where it comes from
There are two ways of measuring luggage and airlines use one or the other. The European way gives three maximum dimensions and you have to meet all three separately; the American way gives a linear sum, and there a long narrow bag can pass where a cubic one of the same volume cannot. The limits are not built in here because they change every season and differ by airline and even by fare within the same airline: you copy them from the website into the form.
How to work it out by hand
- Measure the bag with wheels and handle fitted, because they count too
- Compare each dimension with its limit
- Add the three together and compare with the linear limit if the airline uses one
- Check the weight separately
What is worth knowing
Measurements are taken with wheels, handles and outside pockets included: the gauge at the desk does not forgive the three centimetres of a telescopic handle. A soft bag passes where a hard one does not, because it gives in the gauge, but it also bulges when packed and can end up bigger than it was in the shop. If you are tight on one dimension, look at the margin left: under a centimetre it is not worth the risk, because hand luggage that does not fit gets checked at the gate, which is the most expensive way there is to check a bag.