The standard carry-on allowance is about 45 liters of box: 22 x 14 x 9 inches is 2,772 cubic inches, and one liter is 61.024 cubic inches, so the box holds 45.4 L. A personal item is roughly 25 to 35 liters depending on the carrier. If you only came for a number, those are the two.
The number that matters more is the one nobody prints: a bag sold as “45L” will not fit a 45-liter box, and a bag sold as 40L usually measures somewhere between 39 and 52 liters of box depending on how round it is. Capacity describes the space inside; the airline measures the rectangle outside. Getting those two confused is the single most common reason a bag that “should” fit does not.
Every allowance we track, converted
These are the carriers’ own published limits, converted at 61.024 cubic inches per liter (and 1,000 cubic centimeters per liter for the metric ones).
- 22 x 14 x 9 in = 45.4 L. American, Delta, United, JetBlue and Alaska. Delta states the same limit as 45 linear inches; Alaska counts wheels and handles.
- 24 x 16 x 10 in = 62.9 L.Southwest and Frontier — nearly 40 percent more box than the standard. Frontier also caps the bag at 35 lb.
- 22 x 18 x 10 in = 64.9 L. Spirit, which is unusually wide and deep.
- 55 x 40 x 20 cm = 44.0 L.Ryanair’s priority cabin bag, 10 kg. Its 20 cm depth is about 7.9 in, more than an inch tighter than the US standard.
- 56 x 45 x 25 cm = 63.0 L.easyJet’s large cabin bag, up to 15 kg — a roomier box than the US majors allow.
- 55 x 40 x 23 cm = 50.6 L. Lufthansa, with an 8 kg cabin weight limit that does more work than the dimensions.
- Personal items:United 17 x 10 x 9 in = 25.1 L; JetBlue 17 x 13 x 8 in = 29.0 L; American, Spirit and Frontier 18 x 14 x 8 in = 33.0 L; Southwest 18.5 x 8.5 x 13.5 in = 34.8 L. Delta and Alaska publish no personal-item box at all — it simply has to fit under the seat.
Note what that spread does to shopping advice. A bag that is a comfortable carry-on on Southwest is over on Ryanair by 19 liters of box. There is no such thing as a universally compliant 45L bag, which is why we size pages to the standard 22 x 14 x 9 in box and name the exceptions. The full table is in our carry-on size guide.
Why a 40L bag is not 40 liters of box
Take the three published numbers of any pack, multiply them, and divide by 61.024. That is the box the airline measures. Compare it to the capacity on the label and the gap is the shape of the bag:
- Osprey Farpoint 40— 21 x 15 x 10 in is 51.6 L of box for a 40L bag. It wastes 11 liters of rectangle on curves and harness, which is also why it is over the standard box on two dimensions.
- Osprey Fairview 40— 21.3 x 13.8 x 9 in is 43.3 L of box for the same 40 liters. A slimmer frame, a tighter rectangle, and inside the allowance.
- Nomatic Travel Bag 40L— 21 x 14 x 9 in is 43.4 L of box. A squarer, suitcase-like shape uses its rectangle efficiently.
- tomtoc 40L— 20.5 x 13 x 9 in is 39.3 L of box, which is less than its stated 40L capacity. That is a reminder that stated capacities are marketing figures as much as measurements.
- Cotopaxi Allpa 35L— 22 x 12 x 10 in is 43.3 L of box for 35 liters of bag, so it is nearly as large on the outside as a 40L Nomatic.
- Peak Design Travel Backpack 45L— 22 x 13 x 9.5 in standard is 44.5 L of box, and 22 x 13 x 11 in expanded is 51.6 L. Peak Design is explicit that 35L is the carry-on mode and 45L is not.
The practical lesson: two bags with the same number on the label can be nine liters apart on the outside, and the one with the smaller box is the one that clears a sizer. Dimensions first, liters second — which is how our 22 x 14 x 9 in roundup is built.
What a liter figure actually measures
Nobody audits capacity claims. Reputable brands measure the usable volume of the main compartments, usually by filling them; budget listings frequently quote a number that cannot be reconciled with their own dimensions. Two examples from our own registry, so you can see the range:
- The Pacsafe Metrosafe X 20Lpublishes 17.7 x 11.8 x 5.1 in, which is 17.5 L of exterior box — smaller than the 20 liters on the label. Interior volume cannot exceed exterior volume, so the capacity figure is generous.
- The Vancropak 40Lpublishes 19 x 13 x 7 in standard, which is about 28 L of box, and 19 x 13 x 9 in expanded, which is about 36 L. The “40L” is not supported by either figure, and we say so on its card.
Neither bag is a bad buy for it. But it does mean the liter figure is the least reliable number on a listing, and the one you should lean on least when the question is whether the bag will fly.
How to use the conversion when you shop
- Work in the airline’s units.Get the bag’s three dimensions and compare them to your carrier’s three. Volume is a sanity check, not the test.
- Check depth first. It is the dimension bags fail on most and the one a rigid sizer catches most reliably.
- Treat capacity as a packing figure.Liters tell you how much fits in the bag, which is the right question for trip length — see what size travel backpack.
- Use expanded numbers where they exist. An expandable bag has two boxes, and only the smaller one is a carry-on.
- Measure soft bags packed. A half-full clamshell measures less than its published depth; a stuffed one measures more than you hoped.
Three bags, one per class, with the box math shown
Having set out what to check, here is what each capacity class looks like in a real bag. Prices come from the live price layer with the date they were read.
#1Personal-item class: 20L on the label, 17.5 L of box by its own dimensions
Pacsafe Metrosafe X 20L Anti-Theft Backpack
The real anti-theft pick — not a hidden pocket, but a system: slash-resistant fabric, locking zippers and RFID-blocking pockets. A 20L daypack for pickpocket-heavy cities where you want to stop worrying about your bag. Priced accordingly.
Strengths
- +Slash-resistant paneling and lockable zipper pulls — actual security, not just a hidden pocket
- +RFID-blocking pockets for cards and passport
- +Comfortable, low-key 20L daypack
Trade-offs
- −Security hardware adds weight and cost versus a plain daypack
- −20L is a day bag, not a travel main-bag
| Capacity | 20 L |
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| Weight (empty) | 1.87 lb |
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| External dimensions | 17.7 x 11.8 x 5.1 in |
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| Carry-on fit (our read) | Personal-item sized |
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| Laptop sleeve | 16 in |
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| Material | 750D recycled polyester, eXomesh slashguard |
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| Warranty | Pacsafe 5-year limited warranty |
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Fit note. This is what 'anti-theft' should mean: slash-resistant fabric and locking zippers, not a marketing sticker. Overkill for a quiet trip; exactly right for a crowded market or a night train.
Specs read from the product listing, on September 7, 2026. “Not published” means the maker does not state that figure.
#235L class: 43.3 L of box, two inches of width margin, one inch over on depth
Cotopaxi Allpa 35L Travel Pack
The organizer's carry-on. A full clamshell with a mesh-divided main compartment, tuck-away straps, and a burly weatherproof shell — at 22 x 12 in it is two inches narrower than the standard sizer, though its published 10-inch depth is an inch over. Cotopaxi's warranty is the quiet reason it holds value.
Strengths
- +Split clamshell with zip mesh dividers keeps a whole trip organized
- +Two inches narrower than the sizer, where most 40L packs run wide
- +Weatherproof recycled-nylon shell and Cotopaxi's Guaranteed for Good warranty
Trade-offs
- −Heavier and pricier than plainer 35L packs
- −Bold colorways aren't for everyone (subtler options exist)
| Capacity | 35 L |
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| Weight (empty) | 2.94 lb |
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| External dimensions | 22 x 12 x 10 in |
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| Carry-on fit (our read) | Carry-on (check strict carriers) |
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| Laptop sleeve | 15 in |
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| Material | 100% recycled 840D TPU-coated nylon with ballistic ripstop nylon panels |
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| Warranty | Cotopaxi Guaranteed for Good (lifetime of the product) |
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Fit note. Cotopaxi publishes 22 x 12 x 10 in: two inches inside the standard 22 x 14 x 9 box on width, one inch over it on depth. That shape clears US majors packed sensibly and fits a narrow sizer better than a 40L, but a hard-packed Allpa is not automatically compliant — measure it. The clamshell's mesh dividers are what set it apart from a plain roll-top.
Specs read from the product listing, on July 21, 2026. “Not published” means the maker does not state that figure.
#340L class: 39.3 L of box — the smallest exterior rectangle of any 40L pack we cover
tomtoc Navigator-T66 40L Travel Backpack
A lot of well-built 40L for the money. A structured clamshell with a protective laptop compartment and a lifetime warranty that undercuts the premium brands by a wide margin. The trade-off is a simpler harness — fine for airport-to-hotel, less so for long walks under load.
Strengths
- +Structured clamshell and a well-padded laptop compartment
- +Lifetime warranty at a mid-tier price
- +Clean, business-appropriate looks
Trade-offs
- −Simpler hip belt and back panel than an Osprey — less comfortable under a heavy load
- −At 40L, the same strict-carrier caveat applies when packed full
| Capacity | 40 L |
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| Weight (empty) | 2.65 lb |
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| External dimensions | 20.5 x 13 x 9 in |
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| Carry-on fit (our read) | Carry-on (check strict carriers) |
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| Laptop sleeve | 17.3 in |
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| Material | Recycled 1680D PET fabric, YKK zippers |
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| Warranty | 1-year (2-year with registration) |
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Fit note. The tomtoc's strength is protective tech carry at a mid price; its weakness is the suspension. If you'll walk far with it loaded, budget for the comfort gap versus a hip-belted Osprey.
Specs read from the product listing, on July 21, 2026. “Not published” means the maker does not state that figure.
The Metrosafe X is the clearest illustration of the whole guide: at 17.7 x 11.8 x 5.1 in it is inside United’s 17 x 10 x 9 in personal-item box on depth and over it on height and width, which is exactly the kind of detail a liter figure hides. The Allpa 35L shows how a deep, narrow 35L pack can occupy almost as much rectangle as a 40L. The tomtoc is the counter-example: the smallest box in its class, which is why it keeps appearing on our dimension-first pages.
The limits of this answer
Every airline figure above is from the carrier’s own baggage page and every bag figure is from the maker or the listing, all listed in Sources. Conversions are arithmetic, so they are exact; the capacity claims they are compared against are not, because no standard governs them. Airlines also change allowances without notice, so check your carrier before you fly. What we will and will not assert is in how we choose.