The topic of luggage sizes can be very confusing. I’ve done my best to simplify things, but keep in mind that the gate agents and flight attendants you encounter during a trip will be the ones interpreting an airline’s policy.
Luggage is usually sized by height x width x depth, or by linear inches, which is a total of those three dimensions. For example, a piece of luggage with the size of 29” x 19.5” x 12.5” can also be referred to as being 61 linear inches.
There are different rules for luggage sizes, depending on whether you’re looking at a bag to carry-on to the plane with you or one that you will check with the airline before boarding.
Carry-On Luggage Sizes
Unfortunately, domestic and international carriers don’t use the same luggage sizes to describe carry-on bags. Therefore, the responsibility is yours to ensure that your carry-on bags are an acceptable size. Here's more detail on overhead compartment carry-on size restrictions.
Keep in mind that luggage must be smaller than a carry-on size to fit under the seat in front of you on a flight. See Carry-On Luggage Reviews.
Domestic Carry-On Bag Size Restrictions: Most of the U.S. airlines restrict a carry-on bag to 22” x 14” x 9”, for a total of 45 linear inches. But wait, as of this writing, Spirit Airlines’ restriction is 22” x 18” x 10”, Frontier Airlines, JetBlue Airways, Southwest Airlines and Virgin America use 24” x 16” x 10”, and Alaska Airlines is 24” x 17” x 10”.
What you'll find in customer reviews is that some people have no problem with a carry-on that is over the 45 linear inch standard, and others end up checking the bag at the gate. To be absolutely certain of a trouble-free check-in, the only safe route is to stick to the 45 linear inch standard for most U.S. airlines.
What Carry-On Luggage Size to Buy: If you will always fly with Southwest Airlines, for example, then purchasing carry-on bags that follow their restrictions may work for you. Is it possible that you might get away with a carry-on size that is one inch larger in one of its dimensions that the regulation? Possibly, although some airlines have "sizer" boxes they'll ask you to put your bag in at the gate to ensure it's an approved size.
On the other hand, there are airlines like Delta that put a “sizer” in their gate areas and if your bag can’t fit in it, you’ll be forced to check the bag at the gate.
Logically, you’ll have more flexibility if you buy the smallest acceptable carry-on size if you may travel via more than one airline. Also, keep in mind that some airlines like US Airways and Virgin Atlantic have weight restrictions, also. So, be sure to check before you fly.
International Carry-On Bag Size Restrictions: In the International arena, things get even more complicated. Here are just a few of the size restrictions: 21” x 15” x 9” or 21.5” x 13.5” x 7.5” or 21.6” x 15.7” x 7.8” or 24” x 16” x 10”. As you can see, if you’re flying an International carrier, it’s difficult to make any generalizations. Best to check with the airline before your trip.
Checked Luggage Sizes
Evidently, any bag larger than the approved carry-on sizes must be checked with the airline. But, there are still rules. Anecdotal evidence suggests that it's easier to get a checked bag that is 1" over the maximum size checked, but it's still not guaranteed. See Checked Luggage Reviews
Domestic & International Checked Luggage Size Restrictions: In general, domestic and International airlines define standard checked luggage as being 62 linear inches in size and up to 50 lbs in weight. That linear size equates to a size such as 29” x 19.5” x 12.5”, which is 62 linear inches. As always, you’ll need to check with the airline for specifics.
What Checked Luggage Size to Buy: In general, you’re probably safe with luggage that is 62 linear inches or less, although there may be exceptions.
Maximums also vary. If something you want to check is over 115 linear inches, United will refuse it. If it’s over 80 linear inches, Delta will refuse it.
If you stay with a standard-sized piece of luggage, you’re probably safe. Whether or not you’ll pay an additional fee for checked baggage depends on your airline and the type of fare you purchased. However, If you need to check something that is oversized or overweight, be prepared for large “overage” fees. Fees can add hundreds of dollars to your flight, so it’s wise to be careful when packing.