Linear inches is a term invented by the airline industry to measure baggage. The size of an item in linear inches is the sum of the length plus the width plus the height of the item. A 20-by-20-by-5-inch suitcase, a 1-by-11-by-4-inch painting and a 1-by-1-by-43-inch fishing rod are all the same size in terms of linear inches. Size restrictions are different for different classes of tickets and for different airlines, but the one constant is that airlines measure baggage in linear inches.
Verify that your suitcase is a "box" with six flat surfaces that meet at right angles. When a suitcase has rounded corners, measure to where the corners would be if they were not rounded. Some suitcases will have handles that stick out even after you have pushed them in as far as they will go. With those suitcases, measure the object as if it was a box without protrusions.
Measure the length, the width and the height of the suitcase. Add these three measurements together to get the size of the object in linear inches.
Consult your airline to see if your baggage meets airline restrictions. Typically, airlines allow two checked bags and those bags cannot exceed 62 linear inches in size. Airlines usually allow one carry-on bag that does not exceed 45 linear inches.
The volume varies greatly for containers that have the same measure in linear inches. For example, a 15-by-15-by-15-inch bag has a volume of 15 x 15 x 15 = 3,375 cubic inches. A 1-by-1-by-43-inch bag has a volume of 1 x 1 x 43 = 43 cubic inches. Both are 45 linear inches, but the volumes are very different. To get the largest volume, make sure that length, width and height measurements are as close as possible to each other. To convert to centimetres, multiply the limit in inches by 2.54 and then measure your suitcase in centimetres. For example, 62 inches is 62 x 2.34 = 157.48, which rounds to 157 centimetres. Measure the height, width and length in centimetres and then add the three measurements. If your total is less than 157 centimetres, then the object is less than 62 linear inches.
Most airlines require that carry-on bags fit under the seat or in the overhead compartment. For example, a 1-by-1-by-43-inch item might be within the linear inches requirement, but the item would be almost 4 feet long. Check with the airline before you arrive at the terminal if you have any questions about carry-on bags.