Measure a carry-on including its wheels and handles

Measure a carry-on after packing it, at its widest height, width, and depth, including wheels and handles. Compare each dimension with the current rule of every operating airline on the itinerary. Then make a separate security check for liquids, batteries, and restricted items.

The airline sets the usable size

The Federal Aviation Administration says 45 linear inches — height plus width plus depth — is the maximum for most airlines, but it also tells travelers to check the airline's own number and size rules before packing. An airline can be stricter, and a smaller aircraft can have less overhead space.

Use the 45-inch figure as orientation, not permission. FAA guidance says to check with the airline for the maximum size and number of carry-on items. On a codeshare, find the operating carrier shown for each flight rather than relying only on the company that sold the ticket.

For a current example, Delta lists a maximum of 22 by 14 by 9 inches (56 by 35 by 23 centimeters), with a combined limit of 45 linear inches. Delta explicitly includes handles and wheels in those measurements. Its page also warns that some Delta Connection aircraft with 50 seats or fewer have limited overhead space and may place larger items in the cargo bin.

Measure the packed exterior

Soft bags bulge, expansion zippers add depth, and an overfilled front pocket can turn a nominally compliant bag into a larger one. Pack the bag as it will travel and stand it on a level floor.

Measure three lines:

1. Height: floor to the highest fixed point, including wheels and a non-retracting handle.
2. Width: the widest side-to-side points, including rigid pockets or attachments.
3. Depth: front to back at the deepest bulge, including the expanded section.

Compare each number, not just the sum. A bag measuring 23 by 13 by 8 inches totals 44 linear inches, but it still exceeds an airline's 22-inch height limit. Leave a little room for measurement differences and anything you might add on the return trip.

Keep the security check separate

Bag size is an airline rule; checkpoint contents are a security rule. Passing one check does not establish the other.

TSA's travel checklist limits carry-on liquids, gels, and aerosols to containers of 3.4 ounces (100 milliliters) or less inside one clear, quart-size, zip-top bag, with one bag per passenger. If you carry medication or another necessary liquid, check TSA's current item guidance before packing it in the ordinary bag.

Some objects have cabin-specific rules. FAA tells passengers who must gate-check a bag to remove spare batteries and lithium-battery devices such as power banks and vapes and keep them in the cabin. Check unusual equipment in the FAA PackSafe chart and TSA's “What Can I Bring?” tool before leaving home.

Run a four-part door check

Keep this note with the suitcase:

• Operating airline and current carry-on allowance checked.
• Packed exterior measured over wheels, handles, pockets, and bulges.
• Liquids placed in compliant containers and the applicable pouch.
• Power banks, spare batteries, medication, and special items reviewed separately.

Even a correctly measured bag may be gate-checked when bins fill. Put medicine, identification, keys, and battery-powered essentials in a smaller personal item that can stay with you. The tape measure solves the size question; the separate contents check keeps a last-minute gate change from creating a new one.