What Size Backpack Counts as a Carry-On?
Airlines regulate inches, not liters. A 40L pack can fit every US carrier or almost none, depending entirely on how that volume is distributed.
We have not tested this gear. Every figure here is compiled from published manufacturer specifications and published airline policies, and the arithmetic is public so you can check it. This page contains affiliate links — see the affiliate disclosure.
22 × 14 × 9 in is the standard carry-on box — published by American, Delta, United, Alaska, JetBlue, Hawaiian and Breeze. Southwest and Frontier publish 24 × 16 × 10 in. Airlines regulate dimensions, never liters, so a “40 L carry-on backpack” is a marketing claim rather than a compliance one.
| Published box | Carriers | Practical read |
|---|---|---|
| 22 × 14 × 9 in | American, Delta, United, Alaska, JetBlue, Hawaiian, Breeze | Buy for this and the pack fits everywhere in our table |
| 22 × 16 × 10 in | Allegiant | Roomier, but the carry-on is a paid extra |
| 24 × 16 × 10 in | Southwest, Frontier | 39% more volume — and over the limit on seven carriers |
| 24 × 16 × 11 in | Sun Country | The most generous published box, on a paid carry-on |
Why the liter figure is not the rule
No airline publishes a capacity limit. Not one. Every published carry-on rule in our table is three linear dimensions, measured externally with straps, frame and handles included — because that is what a gate sizer physically is.
So a pack sold as a “carry-on backpack” at 40 L is telling you about its interior, and the gate is asking about its exterior. Those are related but not the same, and the relationship varies by design.
| Pack | Capacity | Published size | US carriers it fits |
|---|---|---|---|
| Tortuga Travel Backpack Pro 40L | 40 L | 22 × 14 × 8 in | 11 of 11 |
| Osprey Fairview 40 | 40 L | 22 × 14 × 9 in | 11 of 11 |
| Osprey Farpoint 40 | 40 L | 21.7 × 13.8 × 9.1 in | 4 of 11 |
| Cotopaxi Allpa 35L | 35 L | 22 × 12 × 10 in | 4 of 11 |
| Peak Design Travel Backpack 45L | 45 L | 22 × 13 × 9.5 in | 4 of 11 |
Three 40 L packs; eleven, eleven and four carriers. And a 35 L pack that fits fewer carriers than a 45 L one. The liter figure predicts almost nothing.
Depth is what fails
Look again at that table and note where every failure sits. Not one of those packs is too tall or too wide — three of them are simply too thick.
There is a structural reason. Height is constrained by your torso and width by your shoulders, so a designer wanting more capacity adds depth. It is the cheapest liter available and it is the dimension the standard box is tightest on: 9 in against 22 and 14.
Depth is also the dimension that grows as you pack. An empty pack at 9 in is not 9 in with a week of clothes in it unless you use the compression straps. Which means:
- Shop published depth first. 8 in is excellent margin, 9 in is standard, 10 in is over.
- Compression straps are a compliance feature, not a comfort feature. A pack that cinches is a pack whose depth you control at the gate.
- Rigid clamshells do not compress. A structured pack at 10 in is 10 in at the sizer. A soft top-loader at 9.1 in is not.
- Measure it packed. The honest method for a soft bag is loaded and strapped, not empty on a spec sheet.
So what capacity should you buy?
Capacity does not decide compliance, but it does decide what you can take. The working bands:
| Capacity | Trip length | Fit risk |
|---|---|---|
| 18–30 L | A personal item, or two to three days | None as a carry-on — but check the personal-item box, which is tighter |
| 30–35 L | Three to five days, or a week with laundry | Low — comfortably inside the standard box |
| 35–40 L | A week, packed deliberately | Depends entirely on shape |
| 40–45 L | A week with bulk — coat, boots, camera | High — usually over on depth |
| Over 45 L | Longer trips | Not a carry-on on any carrier publishing 22 × 14 × 9 |
If you are choosing between two packs of similar capacity, the one with less published depth is the safer purchase every time. The full inches-to-liters arithmetic explains why the standard box is 45.4 L of geometry and why a real pack lands around 40.
If you want the free bag instead
A carry-on backpack and a personal-item backpack are different products with different limits, and the personal-item box is much tighter and much more variable. American, Frontier and Allegiant publish 18 × 14 × 8 in. United publishes 17 × 10 × 9 in, which is 24% less volume.
On the four US carriers that charge for the overhead bag, that free box is the interesting purchase. The personal-item picks are here and the per-carrier limits are here.
Packs whose published dimensions clear every carrier
Each of these satisfies the published carry-on limit at nine or more of the eleven US carriers we track, checked on all three sides.
Osprey Fairview 40 Travel Pack
22 × 14 × 9 in · fits 11 of 11 US carriers
Shorter torsos, and anyone the Farpoint's shoulder straps do not suit
Tortuga Travel Backpack Pro 40L
22 × 14 × 8 in · fits 11 of 11 US carriers
Readers who want the tightest published depth in a 40 L bag
Not sold on Amazon#ad How we earnEagle Creek Cargo Hauler Duffel 40L
22 × 12.5 × 9 in · fits 11 of 11 US carriers
A duffel that will actually pass a gate sizer
Osprey Daylite Duffel 45
21 × 14 × 9 in · fits 11 of 11 US carriers
Maximum published volume that still clears a 22 × 14 × 9 in sizer
Full rankings: carry-on luggage · carry-on backpacks · carry-on duffels