Forget flight delays, narrow seats, lukewarm meals and other perils of air travel: They're nothing when compared to the sickening feeling of seeing your cute little duffel bag rejected from the carry-on sizer, or watching your floppy, extra-large duffel get shuffled into the "oversize baggage" line. Nobody cares about your duffel bag's feelings, of course – it's the extra fees that hurt. But you can avoid them with the help of a measuring tape, a little common sense and a few minutes of extra packing time.
Airline Luggage Rules
Although each airline sets its own rules for how large both checked and carry-on bags can be, they're all dealing with the same types of aircraft. So it's little surprise that a fairly standardized set of sizing guidelines has emerged, purely by accident. As a general rule, most airlines allow checked bags that measure up to 62 linear inches. Linear inches means the sum total of all three dimensions: length plus width plus depth. Sizing for carry-on bags varies more by airline, but maximum sizes of 22 to 24 inches high, 14 to 17 inches wide and 10 inches deep are quite common.
Ultimately, what does and doesn't make it onto a plane as carry-on luggage is up to the discretion of the airline staff. So although you could technically squash your extra-large duffel into the carry-on sizer at the gate, you'll have much better odds of success if you take a smaller duffel as a carry-on, or use a strap to squash the big duffel down to a more acceptable size.
Measuring Your Luggage
Because each airline makes the rules for luggage sizing quite clear, it's usually easy to stay within those guidelines; just measure your bags at their widest point for each dimension and use that to calculate their total size. That gets a little more complicated when you're dealing with something floppy like a duffel bag, which measures differently depending on how tightly you've packed it and whether you're holding it or have set it down on the ground. But the same general rules, plus a little common sense, apply: Just measure the bag while it's laid flat on the ground, because that's how you'll be asked to present it at the airline counter.
Floppy duffel or not, if you're carrying a bag on, make sure you follow the TSA's guidelines and security measures for liquids.
Measuring Floppy Duffel Bags
Make sure you're measuring between the duffel bag's widest points for each dimension. For example, when you measure end to end, don't forget to account for how much the zippers or handles stick out, and when you're measuring the height of the bag, remember to account for the height of handles too. Your perspective matters, as well. You'll want to stand directly over the duffel to measure its length and squat down near it – or place it on a table – to get a good eyeline to the height measurement.