A linear inch is a unit of measurement that corresponds to one-twelfth of a foot. A standard inch is the same as a linear inch, because inches themselves are units of linear measure. Linear measures are one-dimensional, measuring length or distance along only one geometrical axis.
By contrast, square inches, square feet and other square measurements are two-dimensional measures. Square measurements incorporate length and width, and are found by multiplying one by the other. Linear measurements, on the other hand, can be found simply by using a ruler or similar tool. Linear inches are used to express the perimeter of rooms, the height of people or objects and the length of small objects like a shoelace or a sheet of paper.
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.