Liter numbers are a poor way to choose a carry-on backpack. Outside dimensions decide whether it fits the airline’s box; the harness decides whether you will want it on your back; and the opening determines how much unpacking it takes to reach a laptop or jacket.
Disclosure: this comparison uses current dimensions, weights, layouts, and materials from the manufacturers. We have not carried these packs on a trip, so comfort is discussed as a fit question—not a lab score.
Four travel backpacks compared
| Backpack | Size / weight | Capacity | Reason to consider it / catch |
|---|---|---|---|
| Osprey Daylite Carry-On Travel Pack 44 | 21.5 × 14 × 9 in 2.33 lb | 44 L | Common U.S. size fit / no adjustable torso |
| Cotopaxi Allpa 35 | 22 × 12 × 10 in 2 lb 15 oz | 35 L | Divided suitcase layout / too deep for a strict nine-inch box |
| Thule Aion 40 | 20.5 × 13 × 9.1 in 3.19 lb | 40 L | Shorter profile and laptop space / hip support sold separately |
| Patagonia Black Hole MLC 45 | 22 × 14.5 × 7.6 in 3 lb 10 oz | 45 L | Three carry modes / half-inch too wide for a strict 14-inch limit |
Manufacturer figures were checked July 16, 2026. Capacity is not directly comparable across brands, and a soft bag can exceed its listed depth when overpacked.
Which one solves your problem?
Osprey lists the Daylite at 21.5 × 14 × 9 inches and 2.33 pounds. Those dimensions stay within the 22 × 14 × 9-inch box used by several large U.S. airlines. It has a full-zip opening, internal tie-downs, water-bottle pockets, and rear laptop access.
The shoulder harness has a sternum strap and AirScape back panel, but the torso length is not adjustable. The dimensions say nothing about fit on your back. During the return window, load it with the weight you normally pack and walk long enough to notice rubbing or pressure.
The Allpa opens around three sides and divides clothing into one large, one medium, and one small internal section. Shoulder straps and the hip belt tuck away, while the updated design adds a bottle pocket, luggage pass-through, and a sleeve for a 15-inch laptop.
Its 22 × 12 × 10-inch shape is narrow but deep. Ten inches exceeds the nine-inch limit used by several large U.S. carriers. Because the shell is soft, careful packing may reduce bulge; that flexibility is not permission to ignore the published rule. It also holds nine fewer stated liters than the Daylite.
At 20.5 inches tall, the Aion is the shortest pack in this comparison. It includes a 17-inch laptop compartment, internal compression, a bottle pocket, and a removable rolltop pouch for separating damp or dirty items.
Thule lists the depth at 9.1 inches, a small overage for a strict nine-inch box. The compatible Aion sling doubles as a hip belt, but it is sold separately; without it, more of a heavy load stays on the shoulders. The waxed polyester surface will also look and feel different from a conventional smooth nylon pack.
The MLC is the largest option here. It can be carried as a backpack, shoulder bag, or briefcase-style case, and the hip belt doubles as the shoulder strap. The 45-liter main compartment has internal organization and a separate laptop panel for most laptops up to 19 inches.
At 22 × 14.5 × 7.6 inches, it is slim but half an inch wider than a common U.S. limit. Forty-five liters also invites overpacking. It makes more sense for a traveler who wants one large soft case and has checked the specific airline than for someone looking for a conservative international bag.
Fit matters more than a gender label or liter count
Shoulder straps should wrap without cutting into the neck. A hip belt, when present, should land on the hip bones rather than the waist or ribs. The sternum strap should move high or low enough to sit comfortably.
Torso length is the distance from the prominent bone at the base of the neck to the top of the hip shelf. A fixed harness can fit one traveler beautifully and another badly. Manufacturer height ranges are only a starting point.
Test the pack loaded, not empty. Include a laptop, water bottle, shoes, and toiletries, then climb stairs and bend as you would on a train or airport shuttle. Return it if the load pulls backward, the straps rub, or the hip belt cannot sit where it should.
What to check before buying
- Airline fit: compare packed dimensions with every operating carrier.
- Opening: a clamshell is easy for clothing; a top loader can be better for moving between stops.
- Laptop position: a rear sleeve carries close to the body but can complicate access when the bag is packed tightly.
- Stowable straps: useful when a pack is gate-checked or placed on a luggage belt.
- Empty weight: important on an international weight limit and during long walks.
- Compression: helps control the load, but cannot make rigid contents smaller.
A 40-liter pack is generally an overhead bag, not a personal item. For something designed around the under-seat allowance, see our personal-item bag comparison. Travelers looking for adjustable women-specific harnesses can start with the travel backpacks for women guide.
Frequently asked questions
What size backpack can be a carry-on?
It depends on the airline. Many large U.S. carriers publish a maximum near 22 × 14 × 9 inches, while international airlines may use different dimensions and weight caps.
Is 40 liters too big for carry-on travel?
Not automatically. Shape and packed outside dimensions matter more than liters. A tall or deep 35-liter bag can fail where a carefully shaped 40-liter bag fits.
Can a carry-on backpack be gate-checked?
Yes. Tuck away straps when possible and move medicine, batteries, travel papers, and valuables into a smaller bag before boarding.
Is a backpack better than rolling luggage?
A backpack keeps both hands free and handles stairs well. A roller keeps weight off the shoulders. The better choice depends on your body, route, load, and need for quick access.
Product specifications checked July 16, 2026.
Sources
- Osprey: Daylite Carry-On Travel Pack 44 specifications (accessed July 16, 2026)
- Cotopaxi: Allpa 35L Travel Pack specifications (accessed July 16, 2026)
- Thule: Aion 40L Travel Backpack specifications (accessed July 16, 2026)
- Patagonia Worn Wear: Black Hole MLC 45L specifications (accessed July 16, 2026)
- REI Co-op: current Patagonia Black Hole MLC 45L specifications (accessed July 16, 2026)
- American Airlines: carry-on baggage rules (accessed July 16, 2026)
These links are here so you can check the details yourself. ARECO receives no payment when you use them.