Linear inches refers to the combined number of inches of an object's height, length and width. The sum is obtained by measuring the height, length and width of a specific object, such as a piece of luggage, with a ruler or tape measure and then adding the results.
The term "linear inches" is one that the airline industry uses to determine whether a piece of luggage is acceptable for their flights. The airlines impose both weight restrictions and linear inches restrictions on all luggage. This include both checked luggage and carry-on pieces.
The airlines have similar standards for linear inch restrictions in most cases. Most airlines limit checked bags to a maximum of 62 linear inches, according to USA Today. This includes any handles or wheels on the luggage, so passengers need to measure these parts of the piece as well. Many airlines also have a restriction for carry-on bags of 45 linear inches, according to the Federal Aviation Administration.
Although the airlines have linear inch limits in place, they often allow a passengers to go over these limits at the cost of a fee. The policies and fees for bags that are larger than the maximum size vary depending on the specific airline.