Check your airline's luggage policies for carry on and checked baggage, as they will vary. The Travel Insider, a website dedicated to reliable travel advice, places a "safe maximum" for carry on bags at 45 inches, "in the form of a 22" x 14" x 9" bag." The allowable dimensions of checked luggage will vary by airline, but the weight restrictions usually will be firmly between 40 and 50 lbs.
Pack your suitcase completely before measuring. Filling exterior pockets and expandable pouches can add on extra inches that may take you over the limit.
Use a yard stick to flatly measure the height, from the bottom of any rolling wheels to the top of a handle or edge. Using a non-pliable measuring device, like a yard stick, will allow you to read the size more accurately without conforming to the contours of your suitcase's edges.
Measure the width across your suitcase with a yardstick without pressing down on any filled pockets.
Gauge the depth of the suitcase by placing a flat object on top of the suitcase and placing the yardstick on the ground below the suitcase. Where the yardstick meets the other flat object will be the measurement.
