In Consumer Reports' 2016 luggage survey of its subscribers, more than half of the 39,000 people who flew in the past year say they use carry-on luggage on some or all of their trips. That’s a whole lot of people who take carry-on bags with them. The reasons? Almost three-quarters do it to save time at the airport, and two-thirds do it for convenience. Nearly half do it to avoid the cost of checking-in luggage.

The down side of a carry-on is that the bag is small. That might not be a problem for an overnight or weekend trip but for longer jaunts you’ll be clamoring for every available inch. Even though different bags can be the same size on the outside—the limit is 22 inches high x 14 inches wide x 9 inches deep for domestic flights on American, Delta, and United—their usable space on the inside can be very different, depending on how it is configured.

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Figuring out how much packing space you have inside is where things get tricky. Many of the manufacturers of the best luggage brands don’t disclose the interior volume and looks can be deceiving.

So how do you measure the packing capacity? The best way is to inspect the bag for certain telltale space-maximizers. If you want the most interior space possible for packing, look out for these attributes:

  • Squared edges. You’ll get more space if the bag has squared edges. Interior volume is sacrificed if the bag comes with curved corners.
  • No protrusions. An exterior protruding pouch reduces total packing space. Compartments accessible from the outside should be configured as inline pockets, not pouches that protrude.
  • Two wheels instead of four. The wheels on a two-wheeler are partially recessed, yielding more overall volume to the bag rather than the wheels. A four-wheeled spinner has less interior space because the externally mounted wheels steal valuable real estate within the bag’s interior.
  • No wheels. If interior space is really important to you, forgo wheels. The wheels contribute to the overall dimensions of the bag and thus reduce potential packing space.
  • Externally mounted handle. Most handles are mounted internally. But if you must be able to harness every square inch of packing space, external is the way to go. The only major brand of luggage that we know of that has an externally mounted handle is Briggs & Riley. This brand also scored among the highest in overall satisfaction among 16 rated luggage brands in a recent survey of 8,218 Consumer Reports readers who told us about their carry-on suitcases.

Bottom line: Don’t make the mistake of thinking that every carry-on bag has the same packing capacity. Being savvy about your bag's interior just may let you take a little more with you next time you travel.