There is no universal personal item size, so the only figure worth planning around is the smallest one on the trip you are actually taking. Lufthansa allows 40 x 30 x 15 cm in every travel class, United allows 9 x 10 x 17 in, and LATAM allows 45 x 35 x 20 cm with pockets, wheels and handles counted in.
That is only half the test. An allowance is a policy ceiling the airline chose; the space under the seat in front of you is a physical cavity that changes with the aircraft, the cabin and sometimes the individual seat. Delta publishes 14 x 15 x 8.8 in of usable floor under a main-cabin A350-900 seat, closer to 10 x 12 x 9 in on a 767-300ER, and no under-seat space at all in parts of its business cabin. A backpack can clear every number on the airline’s website and still refuse to go in.
There Is No Universal Underseat Standard Behind Personal Item Size
Every allowance quoted here was read on the carrier’s own site on 20 September 2026, and any airline can revise its own figures without notice, so the page to trust for your specific flight is always the one your own carrier publishes. That matters more than usual in this corner of travel, because the agency most travellers assume is in charge has explicitly said it is not.
Security screening does not set these numbers. Asked directly what the size restrictions for carry-on bags are, TSA answers that dimensions “vary by airline” and tells passengers to contact the airline about what will fit in the overhead bin or under the seat. No federal rule in the United States fixes a personal item at 45 x 35 x 20 cm or anything else. The number is commercial policy, written by each carrier, and it moves.
So the spread is real, and it is wider than the round figures suggest:
| Airline | Published personal item limit | What the figure includes |
|---|---|---|
| Lufthansa | 40 x 30 x 15 cm | Same in all travel classes |
| United | 9 x 10 x 17 in (23 x 25 x 43 cm) | Must fit under the seat in front |
| LATAM | 45 x 35 x 20 cm | Pockets, wheels and handle included |
| American | 18 x 14 x 8 in (45 x 35 x 20 cm) | Must fit under the seat in front |
Lufthansa’s 15 cm depth is the constraint that catches people. A bag built to LATAM’s 20 cm depth is 5 cm too thick for a Lufthansa personal item allowance before anything goes inside it, and on a two-carrier itinerary the stricter airline governs the whole trip.
This is also where the widely-shared comparison charts stop being safe. Aggregated “personal item dimensions by airline” tables go stale quietly, and at least one popular luggage-brand chart currently lists a Lufthansa figure that contradicts what Lufthansa itself publishes. Read the carrier’s own baggage page for each airline on your ticket, and treat every third-party chart, including this one, as a starting point rather than an authority.
Your Airline’s Personal Item Allowance Is Not the Space Under the Seat
An allowance answers a commercial question: how much cabin baggage will this airline sell you at this fare. It is a maximum the airline is willing to accept, not a measurement of anything on the aircraft. Nothing in a published allowance promises that a bag at the limit will physically fit the floor space at your seat.
The regulation behind cabin stowage makes the distinction plain. Under 14 CFR 121.589, a US carrier may not allow a passenger to board with carry-on baggage unless each article is stowed in an approved location, and the rule is about where an item ends up, not how large the airline said it could be. Approved stowage means an overhead bin, a closet, or the floor under a seat, and the carrier carries the obligation to ensure it happens before pushback.
That produces the gap at the centre of this problem. The airline’s page gives you a number. The aircraft gives you a cavity. The two are set by different people for different reasons, and neither one is calculated from the other. A 45 x 35 x 20 cm personal item allowance and a 25 cm-deep under-seat opening can coexist on the same flight without either being an error, and when they do, the cavity wins.
Practically, this means a compliant bag has two independent tests to pass, and most packing advice only ever describes the first one. The second is the one that decides whether the bag actually goes in.
What Actually Occupies the Space Under an Airline Seat
The floor under a seat is not an empty rectangle. It is a space interrupted by the seat’s own structure and by whatever equipment the airline has bolted into it, and the interruptions are rarely symmetrical from one seat to the next.
The most common obstruction is the in-flight entertainment box. On long-haul aircraft with seat-back screens, the processing units sit in a metal housing mounted on the floor beneath a seat, typically under one or two seats in a three-seat block rather than all of them. A box of that kind removes several centimetres of both width and height from the space available to the passenger behind it. Seat legs and the horizontal structural bar that ties them together take more, which is why the usable opening is almost always narrower at the bottom than the seat looks from above.
Cabin baggage guides written from actual measurement sessions, including the long-running under-seat measurement project at Dog Jaunt, report the same pattern across many aircraft: usable depth and width vary seat by seat within a single cabin, and the published figure is the airline’s general case rather than a guarantee for your row. On several narrow-body types the window, middle and aisle positions in the same row each measure differently, and the middle seat is commonly the one that inherits the equipment box.
The practical consequence is that width, not height, is usually what stops a backpack. A soft pack will compress vertically under moderate pressure. It will not compress past a steel IFE housing or a seat leg, and a bag that is 2 cm too wide is simply too wide.
Published Underseat Clearance by Aircraft and Cabin Class
Very few airlines publish under-seat dimensions at all, which is what makes Delta’s aircraft pages unusual and worth reading closely. The figures below are Delta’s own, read on 20 September 2026, and they are stated by the airline as approximate.
| Delta aircraft | Main Cabin under-seat space (approx.) | Notes |
|---|---|---|
| A350-900 | 14 x 15 x 8.8 in (36 x 38 x 22 cm) | Among the most generous published |
| A330-300 | 11 x 16 x 9 in (28 x 41 x 23 cm) | Varies by seat position |
| 767-300ER | 10 x 12 x 9 in (25 x 30 x 23 cm) | Noticeably tighter than the A350 |
| 767-400ER | Differs from the 767-300ER | Same family, different cabin fit |
Two things in that table deserve attention. The first is the spread within a single airline: roughly 15 in of usable width on an A350-900 against about 12 in on a 767-300ER. A bag sized confidently for the first aircraft is not guaranteed on the second, and both fly long-haul routes for the same carrier under the same published personal item allowance.
The second is the variant trap. The 767-300ER and 767-400ER are the same aircraft family, sold under near-identical names on a booking page, and their cabins do not offer identical under-seat space. Knowing you are on “a 767” is not enough. The sub-variant is the thing that determines your floor.
Cabin class changes the picture again, and not always in the direction people expect. Premium cabins with lie-flat seats frequently have no conventional under-seat storage whatsoever, because the shell, the footwell and the seat mechanism occupy the floor. Paying more for the seat can mean having less room for a bag during taxi, takeoff and landing, which is precisely when you need somewhere legal to put it.
How to Test a 20 L to 30 L Backpack Against Both Limits
Before any of this works, a warning about the numbers printed on bags. They are less reliable than almost anyone assumes, and the failure is not confined to small brands.
Osprey’s own specification table for the 20 L Daylite Plus currently lists its dimensions as 9.45H x 11.02W x 18.9D in, which describes a daypack that is 18.9 in deep and 9.45 in tall. No 20 L daypack has that shape. Major retailers list the same bag with the axes the other way round, as roughly 18 x 10 x 9 in with 18.9 in given as the height. The three numbers are consistent across all of them; only the labels disagree. If you match “height to height” from a spec sheet to an airline’s limit, you may be comparing the wrong pair entirely.
The fix is to ignore the labels. Sort your bag’s three measurements from longest to shortest, sort the airline’s three permitted dimensions the same way, and compare them in order. An airline’s under-seat opening does not care which side the manufacturer calls the height, and neither should you.
Step 1: Measure the packed bag, not the empty one
Measure the bag loaded the way you will actually fly it, across its widest points, with any external pocket full and any compression strap pulled as tight as you intend to pull it at the gate. A 24 L pack measured empty and the same pack stuffed with a jacket are different objects. LATAM states explicitly that pockets, wheels and handles count toward the limit, so measure the bulges rather than the seams.
Step 2: Sort both sets of three numbers and compare in order
Write your three measurements longest to shortest. Do the same with each carrier’s published limit. Compare longest against longest, middle against middle, shortest against shortest. Every one of the three must clear, and failing any single axis fails the bag.
Step 3: Check the answer in both units
This step is not optional, and here is why. The Aer City Pack Pro 2 in 24 L is published by its maker as 18 x 12 x 7 in, or 46 x 30.5 x 18 cm. Against American’s personal item allowance of 18 x 14 x 8 in, it passes on all three axes with room to spare. Against the same allowance written in the same airline’s own metric conversion, 45 x 35 x 20 cm, its longest axis is 46 cm and it fails.
Nothing about the bag changed. Eighteen inches is 45.72 cm, which American rounds down to 45 and Aer rounds up to 46, and the rounding decides the verdict. Where a bag sits within a centimetre of any limit, check both units and plan for the stricter reading, because the gate agent’s sizer and the crew’s judgement will not be applying your arithmetic.
Applied to the two packs above, against the limits verified on 20 September 2026:
| Bag (maker’s own figures) | Sorted dimensions | Lufthansa 40 x 30 x 15 | United 43 x 25 x 23 | American / LATAM 45 x 35 x 20 |
|---|---|---|---|---|
| Osprey Daylite Plus 20 L | 48 x 28 x 24 cm | Fails | Fails | Fails |
| Aer City Pack Pro 2 24 L | 46 x 30.5 x 18 cm | Fails | Fails | Borderline |
A popular 20 L daypack sold worldwide for travel misses every one of the four allowances, mostly on its longest axis. That is the honest state of the market, and it is the reason underseat bag dimensions deserve checking before purchase rather than at the gate. For a personal item size bag intended for genuinely international use, building to Lufthansa’s 40 x 30 x 15 cm gives the widest margin, and anything approaching 45 cm on its longest axis should be treated as trip-specific rather than universal.
Seats That Remove Underseat Storage Altogether
Some seats have no usable under-seat space by design, and others have it removed by regulation for part of the flight. Both produce the same outcome for a passenger holding a compliant backpack.
Bulkhead seats are the clearest case. With no seat in front, there is no floor cavity to use, and everything must go overhead for taxi, takeoff and landing. Exit-row seats are governed more strictly still: under 14 CFR 121.589, carry-on baggage must be stowed so that it does not obstruct access to an exit or block an aisle, which in practice means nothing may sit on the floor at an exit row during those phases of flight. Cabin crew enforce this without discretion, and a bag that fits perfectly is still not allowed to stay there.
Lie-flat business and first-class seats are the third category. The footwell, the seat shell and the actuating mechanism take the floor, and storage is redistributed into side consoles or enclosed compartments that may be smaller than the seat’s price suggests. Delta states directly that some of its business cabins have no under-seat storage at all.
None of these seats is a bad choice. They simply change where your bag lives, and if your plan depended on keeping a camera, a laptop or medication at your feet from gate to gate, the seat map is the place to discover that rather than the jetbridge.
Researching Your Own Aircraft and Seat Before You Fly
Two lookups, done in the order below, will settle almost every case before you leave home.
Start with the aircraft on your specific booking, not the route. Your confirmation or the airline’s manage-booking page names the type and usually the variant, and that string is what you carry into every other check. Then read your own carrier’s aircraft page for that exact variant, which is where Delta’s under-seat figures live and where most other carriers at least publish a seat map.
For seat-level detail, the practical tool changed recently. TripAdvisor shut SeatGuru down in November 2025, and the closest replacement is AeroLOPA, whose founder created SeatGuru in 2001 and whose maps are drawn to scale by an architect rather than sketched. To-scale drawing is the relevant property here: it lets you see where the seat legs and equipment boxes actually sit in your row, which is exactly the information a generic seat map omits.
Seat choice is the lever most travellers forget they have. Where under-seat space varies within a row, it is usually the window and aisle positions that keep a clean opening and the middle seat that inherits the entertainment box. If keeping your bag at your feet matters, choosing the seat at booking is worth more than any packing technique.
Happy Day Turismo is a travel agency and sells flights, which means we can assign seats for you as part of a booking; you can also do exactly the same thing yourself, free, on your airline’s own site at the moment you book. Neither route changes the aircraft, and the seat map is identical either way.
When a Compliant Backpack Still Will Not Go Under the Seat
Sometimes the bag is legal, the measurements were right, and it still will not go in. Knowing the recovery options keeps a bad moment from becoming an expensive one.
The most common cause is a regional aircraft. American publishes a specific procedure for this on its carry-on baggage page: on American Eagle regional flights, a bag larger than the personal item dimensions must be valet-checked before boarding, tagged at the gate at no charge, and returned to you on the jetbridge at your destination. American excepts the Embraer ERJ-175 from this, which takes ordinary carry-on bags and offers no valet service. The service costs nothing and is built for exactly this situation, but it only helps if you present the bag before boarding rather than discovering the problem in the aisle.
The second cause is a full cabin. Where overhead space runs out, airlines will move cabin bags into the hold, and the decision is operational rather than a judgement about your bag. Anything you cannot be separated from, including medication, travel documents, valuables and a power bank, should be in a small pouch you can lift out in the few seconds you will be given.
Crew instruction overrides every published figure. If a crew member says a bag cannot stay at your feet, that instruction stands on the day regardless of what any airline page says, and arguing measurements at the door delays the flight without changing the outcome.
One related failure has a different cause and a different fix: a bag that meets published dimensions and still will not pass the metal sizer box at the gate, where tolerances and the box’s own build decide the result. That is the overhead-bin half of the same problem, and it is worth reading separately once this one is settled.
O Que Levar à Mão no Embarque
The two tests a backpack has to pass are set by different people: the airline writes the allowance, the aircraft decides the cavity, and clearing one proves nothing about the other. Before you buy, measure the packed bag at its widest points, sort its three numbers longest to shortest, and compare them in that order against the strictest carrier on your itinerary. Build toward Lufthansa’s 40 x 30 x 15 cm if the bag has to work internationally, and check any borderline result in both inches and centimetres. Do that and the bag stays at your feet; trust a comparison chart’s round number and you find out at the boarding door, when the options left cost time or money.
Frequently asked questions
Can I put my personal item in the overhead bin instead?
Usually yes on a wide-body aircraft with space available, but it is the crew’s call and not a right. On full flights, airlines reserve overhead space for full-size carry-ons and will direct personal items back under the seat.
Does a handbag count as a separate personal item?
On most carriers a handbag, laptop bag or small backpack is the one personal item, not an extra allowance on top of it. Combining a purse and a backpack usually means one of them counts as your carry-on.
What happens if the cabin is full and my bag fits perfectly?
Airlines will still move cabin bags into the hold when overhead space runs out, and a compliant bag is not exempt. Keep medication, documents, valuables and power banks in a pouch you can remove quickly, because you will get very little warning.
Is a rolling underseat bag a better choice than a backpack?
Rarely, for this specific purpose. Wheels and a telescopic handle count toward the published limit on carriers such as LATAM, and they add rigid depth that cannot compress past a seat leg or an entertainment box, which is where most under-seat failures happen.
