Is a 40L Backpack a Carry-On or a Personal Item?

A liter rating is how much clothing and gear the maker says the pack will hold. Airlines measure the packed exterior. The gate box has three sides, and the packed pack has to fall through it, straps and pockets included.

You can't turn 40 L into one true H x W x D. A cube of 40 L is 13.5 inches on a side. A travel pack is a tall, thin rectangle. Our estimate uses the US cabin proportion, 22:14:9, and scales that rectangle until the volume matches. Change the proportion and the inches move.

That's why we never convert three sides into liters and call it capacity on how we measure luggage. Going the other way, liters to inches, is an estimate with the same warning attached.

40 L x 61.024 = 2,441 cubic inches. Osprey prints that same 2,441 cu in figure on the Farpoint 40.

Scale a 22 x 14 x 9 box down to 2,441 cubic inches and you get about 21.1 x 13.4 x 8.6 inches. Every side of that estimate is inside the US major cabin box. The height, 21.1, is 3.1 inches over the 18-inch personal-item height American, Frontier, and Allegiant publish.

Tortuga lists its Pro 40L at a different rectangle: 22 x 14 x 8 inches (55 x 35 x 20 cm). That is the same 40 L of volume, one inch shallower, still 4 inches too tall for under the seat.

Either shape is overhead. Neither shape is a personal item.

A carry-on goes in the bin. A personal item goes under the seat in front of you. A 40 L travel pack is built to the bin. The under-seat job belongs to a smaller personal item backpack cut to the box on your ticket. The full split between the two slots is on personal item vs carry-on.

On Frontier, the overhead bin is a paid add-on and the free bag is the 18 x 14 x 8 personal item. Showing up with a 40 L pack on a bare Frontier fare means buying the carry-on or paying at the gate. United Basic Economy is the same trap on most routes: personal item only, so the 40 L pack is a prepaid carry-on or a $75 gate bag.

Southwest publishes a larger cabin box, 24 x 16 x 10, and no personal-item inches at all; that bag just has to go under the seat. The 40 L pack is still the carry-on on that airline. It isn't the bag you slide under the seat.

Maker listings disagree with each other, which is the point. These are the makers' published exteriors, rechecked before this guide went live. None of them is a packed measurement of a bag in a sizer.

Osprey's 46 L Porter lists a slightly smaller exterior box than Osprey's 40 L Farpoint. Liters and inches are different measurements. The Farpoint 40's listed depth, 9.1 in, is 0.1 in past the 9 in US cabin depth. Empty, that's rounding. Packed, the front panel is what a sizer hits first.

The Farpoint 40 is the 40 L carry-on in our best travel backpacks for men roundup. Treat the hangtag as the starting box, then tape the packed pack.

The Farpoint 55 is a 40 L main bag plus a 15 L daypack. Zipped together, Osprey lists it at 21.7 x 13.8 x 18.1 inches, twice the depth of a 9-inch cabin box. Unzipped, the main bag is the same 40 L carry-on problem, and Osprey pitches the daypack as the personal item.

That split is why people buy the 55. A "Farpoint" search still doesn't tell you whether the bag on your back will fit a sizer. For the two-bag system itself, the Osprey Farpoint 55 review covers the harness, the daypack zip, and what owners report after months on the road.

Figures below are each airline's published cabin and personal-item limits, rechecked on the carriers' own pages before this guide went live. Our carry-on size chart has the rest. A 40 L pack is the cabin bag in every row. It is the personal item in none of them.

Spirit is missing on purpose. It stopped flying on 2 May 2026, so any Spirit sizer rule still circulating describes an airline you can't book.

Ryanair is the European version of the same mistake. The hangtag on a Farpoint 40 is 9.1 in / 23 cm deep. Ryanair's cabin box is 7.9 in / 20 cm deep, and the fare rules behind it are in our Ryanair cabin bag size guide. Compression straps are the only thing that closes that gap, and a sizer doesn't care that the pack was 40 L on the website.

Ignore the liter number on the hangtag and measure the packed pack. Height includes the grab handle. Width includes the side pocket. Depth is the wall-to-front bulge with the harness stowed the way you will stow it at the gate.

That method, for a suitcase, is in how to measure luggage. A backpack adds three extra failure points a spinner does not have: the front organizer, the bottle sleeve, and hanging straps. Stow the harness. Empty the bottle sleeve. Then tape it.

If the packed height is over 18 inches, it is not a personal item. If any packed side is over 22 x 14 x 9, it is not a US major carry-on either, and you are in gate-check territory.

A 40 L backpack is the bag you put in the bin. Buy a second, smaller pack if you need something under the seat. The liter number on the hangtag won't get you through a sizer. The packed exterior will.