Measure suitcase sizes using a tape measure, accurately measuring height, width and depth in inches. When measuring the height for compliance with airport regulations, be sure to include the wheels in the measurement; some manufacturers do not include the wheels in size descriptions.
For future reference, write down the measurements on the suitcase's identification tag. It is important to accurately measure any suitcase you plan to carry on an airplane. Check the airline's website for the dimensions permitted for carry-on luggage. If your luggage does not comply with the requirements, the airline may require you to check your bag, possibly at an additional cost.