Measure linear inches for luggage by adding up the measurements of a suitcase's height, width and depth and comparing the result to an airline's requirements. Airlines typically limit the size of carry-on luggage based on measurements in linear inches.

**Measure the suitcase**Use a tape measure or ruler to measure the height, width and depth of your suitcase.

**Add the measurements**Add together the height, width and depth measurements. The total number is the linear measurement of the suitcase.

**Compare the measurements to the airline's requirements**Check the carry-on requirements of the airline on which you plan to fly. Most airlines allow carry-on luggage with a maximum linear measurement of 45 inches, but some airlines set their own standards.