How to Measure a Backpack for Airlines
Pack the backpack as you'll fly it, stow the straps, and measure the packed exterior. Height reaches the highest point, including the grab handle. Width spans the widest occupied pocket or other protrusion. Depth runs from the back panel to the farthest front bulge. A sizer uses those three sides, not the liter rating or the empty listing.
We rechecked the airline boxes named below on each carrier's own baggage page before this guide went live.
Buying one? How to Measure Luggage the Way Airlines Do
Pack the backpack first
The bag that goes in the sizer is the packed bag. An empty travel pack sits flatter than the same pack with clothes, a laptop, and a bottle in it. Fill the main compartment the way you'll fill it for the flight. Fill the front pocket. Leave the bottle in the sleeve if that's how you'll walk through the airport.
Don't squash the pack to make the tape look better. Measure its natural packed exterior so you can compare it with the carrier's limit.
A rigid, nonexpanded suitcase usually changes less from empty to packed. A soft backpack can bulge at the front pocket, bottle sleeve, and anywhere a jacket is strapped to the outside.
Stow the straps
Shoulder straps that hang off the top add height. A hipbelt that sticks out adds depth. Some carriers put that in writing. Frontier's bag options page measures personal items and carry-ons including handles, wheels, and straps.
Clip the hipbelt around the pack or into its stowaway pocket. Fasten the sternum strap. Tuck or clip the shoulder straps against the back panel so they don't extend beyond the packed exterior. Measure that shape.
If you want the difference in inches, take height a second time with the straps hanging and write both numbers down. Use your tape for that difference. Don't guess it.
The wall-and-book method
A cloth tape can follow the curve of a stuffed front pocket, but that curved path isn't the straight-line exterior depth. Use a rigid tape, a wall, and a hardcover book or a carpenter's square.
Stand the packed pack up with the back panel flat against the wall, sitting on the floor the way it would sit in a sizer, not hanging from the harness.
The wheeled-suitcase version of this job, standing a spinner on its wheels, is on how to measure luggage. Don't use that stance for a backpack unless it has wheels. For a standard backpack, include the grab handle, bottle sleeve, front bulge, and any other protrusion.
- Height. Measure from the floor to the highest exterior point. That point is often the grab handle. On a roll-top it can be the rolled fabric. On a hiking pack it can be an item protruding from the lid. Set the book on that high point, square to the wall, and run the tape from the floor to the underside of the book.
- Width. Measure left to right at the widest exterior point. That is often a bottle sleeve with the bottle still in it, or a side pocket stuffed with a tripod or folded jacket. Put the book against one side and measure straight across to the other.
- Depth. Put the back panel against the wall and measure straight out to the farthest front bulge: the admin pocket, daisy chains, sleeping pad, or jacket strapped on. Put the book against that bulge and measure from the wall to the book without pushing.
| Side | Where the tape goes | What can add to the measurement |
|---|---|---|
| Height | Floor to grab handle, or whatever sits higher | Hanging straps, a roll-top, a bottle in the lid |
| Width | Widest side pocket or bottle sleeve | A bottle left in the sleeve |
| Depth | Wall to the farthest front bulge | A packed admin pocket, a jacket on the outside |
Write the three numbers down while you still have the pack in that stance. Then compare each side to the airline box, longest to longest. A pack that clears height and fails depth still fails.
Empty vs packed
Makers may quote the empty pack, a volume in liters, or a "carry-on size" line that doesn't say packed or empty. The relevant comparison is the packed exterior against the airline's stated limit.
Packing can make the front organizer and bottle sleeve bulge. Hipbelt pads add depth if you leave them flared. Compression straps left loose can let the main compartment barrel out. A removable rainfly or jacket on the daisy chains also adds to the packed exterior.
The bag figures we publish elsewhere on this site come from maker listings and the sources each row names, and we say what we do when a listing leaves a field blank. Those rules are on how we measure bags. For the sizer, the number that matters is the packed one you take yourself.
Liters are not a sizer
Liters describe volume. The sizer tests the height, width, and depth of the packed bag. Two packs with the same liter rating can have different outside shapes: one might be tall and thin, while another might be shorter with a thick harness.
A liter rating doesn't establish whether a travel pack fits an overhead or under-seat box. Compare its packed exterior with the carrier's published dimensions. If you need a smaller starting point, our personal item backpack page lists packs built around under-seat travel.
Which box you are measuring against
Measure the packed pack first, then look up the carrier on the ticket. These are the boxes that come up most often for backpacks.
American Airlines, carry-on page: carry-on 22 x 14 x 9 inches (56 x 36 x 23 cm), including handles and wheels, and it must fit the airport sizer. Personal item 18 x 14 x 8 inches (45 x 35 x 20 cm), and it has to fit under the seat.
Frontier, bag options page: personal item 18 x 14 x 8 inches, including handles, wheels, and straps, up to 35 lb, and "the size of your personal item will be checked during boarding." Carry-on 24 x 16 x 10 inches on the same including-straps basis, also 35 lb. The free bag on a basic Frontier fare is the personal item. The larger pack is a paid carry-on.
Southwest publishes 24 x 16 x 10 inches for the carry-on on its carry-on policy page. That page doesn't publish personal-item dimensions; the personal item must fit under the seat. The rest of the Southwest rules are in our Southwest carry-on size guide.
A 55 x 40 x 20 cm cabin box is about 21.7 x 15.7 x 7.9 inches, so a US-style 22 x 14 x 9 travel pack can exceed it on height and depth. Check the airline row and its dated source before you fly.
For the carrier you are actually flying, use the luggage size chart. It keeps 68 reference rows, and each row names the page the figures came from and the date we last read it.
Carry-on pack or personal item
A backpack may count as an overhead carry-on or an under-seat personal item. The allowance depends on the airline, fare, route, and ticket. The difference between the two slots is laid out in personal item vs carry-on.
If the packed pack fits the personal-item box and goes under the seat, it can serve as the personal item. If it fits only the carry-on box, it can serve as the carry-on where your ticket includes or permits one. If it fits neither, check the airline's checked-bag and airport-fee options.
Some fares include only a personal item, while others also include or allow an overhead bag. Measure against the smaller box when your ticket includes only a personal item, and review the airline's price before the airport if you need to add a carry-on. The two-bag version of this, a spinner plus a pack, is on carry-on and a backpack.
If the tape is over
Cinch the compression straps. Empty the bottle sleeve. Take the jacket off the daisy chains. Restow the harness. Measure again.
Soft fabric may shift, but don't rely on compressing an over-limit pack into the sizer. Frontier's page says personal-item size will be checked during boarding.
If the packed pack is still over after you remove exterior items and restow the straps, review the airline's carry-on, checked-bag, and airport-fee options. Move lithium batteries, medication, travel documents, and other cabin essentials into a bag that can stay with you if the larger pack goes in the hold.
FAQ
Do airlines measure backpacks packed or empty?
Packed. The sizer sees the bag you carry to the gate. An empty listing leaves out the front-pocket bulge, a bottle in the side sleeve, and hanging straps. Measure after you pack, with straps stowed, then compare each side to the published box.
Do hanging backpack straps count?
Yes when they add height or catch the lip of the box. Clip the hipbelt, tuck the shoulder straps against the back panel, and tape that shape. Frontier's published personal-item and carry-on limits include straps, so hanging webbing is part of the bag they measure.
How do you measure a backpack without an airport sizer?
Pack it, stow the straps, stand the back panel against a wall, and use a rigid tape with a book as a square. Height is floor to grab handle. Width is the side pocket. Depth is wall to the front bulge. Compare those three numbers to the airline's published box.
Can I use the liter rating instead of measuring?
No. Liters are volume and do not convert cleanly into a sizer. Tape the packed pack and compare each side with the carrier's published box.
Does a backpack count as a personal item?
Yes if the packed pack fits the personal-item box and goes under the seat. American publishes 18 x 14 x 8 inches. Frontier publishes the same three dimensions, including straps, and says it will check personal-item size during boarding.
Filed in Travel Tips