Measure including the wheels
Nine of the thirteen airlines here state explicitly that wheels and handles are part of the measurement. British Airways says "including wheels and handles". Southwest says "wheels, handles, and any attachments count". Delta says "include any handles or wheels".
This matters more than it sounds, because a bag sold as 55 cm is usually 55 cm of body with wheels underneath, taking the real figure to 58 or 59. The sizer at the gate measures the object, not the label. If you are close to the line, measure the bag yourself with a tape.
Orientation does not save you
The checker sorts your three measurements largest to smallest and compares them against the airline's three sorted the same way. That is deliberate: a sizer is a box, and a bag that fits does so in some orientation. There is no rule requiring the long side to be the one the airline calls length.
What it will not do is let a bag through on a technicality. If your longest side beats their longest, it fails, regardless of how the other two compare.
Two limits most checkers miss
Delta publishes a 45-linear-inch cap — 114 cm across all three sides added together — on top of the per-side maxima. A bag can be inside every individual dimension and still fail it. This checker computes it; no other one I found does.
Singapore Airlines goes the other way and publishes no per-side maximum at all, only that the three sides must total under 115 cm. A long, thin bag can pass there and fail everywhere else, and a squat one can do the reverse.
Things people ask
Should I measure in inches or centimetres?
Centimetres, if the airline publishes them. Several carriers print an inch figure that is rounded in their own favour — Qatar prints 20 × 15 × 10 in for a bag that is really 19.7 × 14.6 × 9.8.
Does the personal item count separately?
Yes, where the airline allows one. Emirates Economy does not — one bag only, which is the most expensive misconception on that carrier.
My bag is 1 cm over. Will they notice?
Sometimes not, and that is not advice this site is willing to give. Gate checks are inconsistent by airport and by day, and the fee when it goes against you is far more than a smaller bag costs.