These restrictions make it increasingly difficult to know exactly what you can take on to a flight in your hand luggage.
The bad news is that each airline sets its own rules on the size and weight of hand luggage that is allowed on a flight. The enforcement of these rules is also dependent on the airline, airport and sometimes just the employee at the gate.
Some airlines give the dimensions as a single measurement such as 45 in, this represents the total of the height, width and length these are usually in standard sets. 45 inches for example usually refers to the safe maximum of 22" x 14" x 9". But beware; some airlines may reject a bag if it is of an equivalent volume but an unusual shape which exceeds any one of these dimensions.
When you are buying a case with the intention of taking on a flight as hand luggage, always check the dimensions, as some cases are described as 'carryon' cases, but may exceed many airline restrictions. It is important to measure the bags yourself as often manufacturers give the inside measurements and exclude any wheels and handles which may add an inch or two.
Some airlines such as Easyjet will have a cage at the check in where you can test your bag before attempting to board with it, although by then it is often a little late.
Once your carryon luggage is on the plane there is no guarantee that it will fit into the overhead locker, or even under the seat in front of you.