Soft vs. Hard Airline Carriers: Why In-Cabin Is Almost Always Soft
People shopping for an “airline carrier” often reach for a rigid plastic kennel, because that is what an animal shipped in cargo travels in. For riding in the cabin with you, that is usually the wrong tool, though “usually” is doing real work in that sentence and the checker below is where it gets pinned down.
In-cabin carriers have to fit under the seat in front of you, and a soft-sided carrier flexes into that space where a hard-sided kennel cannot. That is why the four picks above are all soft-sided, and why six of the eight pet policies we checked either recommend soft-sided or accept nothing else. The two that do not state a preference say so plainly: JetBlue’s page reads “Soft-sided and hard-sided carriers are accepted”, and United’s reads that pets “must travel in either a hard-sided or soft-sided carrier.”
What is not true is that hard kennels are barred from the cabin outright. On August 2, 2026, American, United, Frontier and Allegiant each published a hard-sided in-cabin maximum alongside their soft-sided one, and JetBlue published one figure that governs both types. Alaska and Hawaiian are the exception that people generalize from: their shared pet page says “Hard-sided kennels are not allowed in the cabin.” Delta and Southwest publish no hard-sided figure at all; neither page names the format. Where an airline publishes both, the hard-sided number is usually the shorter of the two, which is the real reason a rigid shell is harder to fly with than a soft one.
So if you are choosing between a soft carrier and a hard kennel for flying in the cabin, the answer is soft, and the size table further down shows exactly what that choice buys airline by airline. If you specifically want a rigid shell for a fearful cat or for car transport, that is a legitimate need, but confirm it against your airline rather than assuming, because on Alaska and Hawaiian it cannot board at all. Our best cat evacuation carriers roundup covers hard-shell top-load kennels (the Petmate and Frisco two-door models) for car and home use, and our vehicle loading and restraints guide covers rigid crash-tested crates for the drive.
Two soft carriers come up more than any others in this decision, and they solve different halves of it: one leads on published crash certification, the other was built around cabin fit. We put them head to head, including the weight caps that rule some households out, in Sleepypod vs Sherpa for an evacuation carrier.
Spec Comparison: In-Cabin Carrier Options
| Carrier |
Type |
Published Ext. Dimensions |
Weight Capacity |
Crash-Tested |
Airline Claim |
| Sherpa Original Deluxe |
Soft-sided |
17“L x 11”W x 10.5“H (Medium) |
Up to 16 lb (Medium linked; Large sizes to 22 lb) |
No cert found |
“Guaranteed On Board” under-seat program |
| Diggs Passenger |
Soft-sided |
20“L x 10.8”W x 11.5“H |
Up to 18 lb |
Yes, CPS 5-star |
Maker markets as carry-on compatible |
| Away Pet Carrier |
Soft-sided |
18.7“L x 10.8”W x 10.75“H |
Up to 18 lb |
Yes, CPS 5-star |
Maker markets as cabin carry-on |
| Sleepypod Atom |
Soft-sided |
17“L x 8.5”W x 10.5“H |
Up to 12 lb |
Yes, CPS 5-star |
Compact under-seat footprint; no airline stamp claimed |
Every figure in this table is cited per-product in the spec tables above and in the sources list at the bottom of the page. Note that all four are soft-sided, which is the “soft vs. hard” answer in one glance: for in-cabin travel, soft is the format six of the eight policies in the checker below recommend or require, with JetBlue and United neutral between the two. What the table cannot tell you is whether these dimensions clear the airline you are actually booking, which is the next section.
Carrier Fit Checker: Will the One You Already Own Board?
Recommending a carrier is half the job. Most people reading this page own a bag already and want to know whether it flies. Here is how to check it, and the answer is less reassuring than the phrase “airline approved” suggests.
Step 1: Measure the carrier, not the listing
Measure the outside of the assembled carrier, loaded, at its widest point. Three numbers: length, width, height.
- Wheels, feet, handles, side pockets and whatever is stuffed in them are part of the measurement. An agent measures the object in front of them, not the shell it started as.
- Height is the tallest point of the zipped carrier with the pet inside, not the number on the listing after the top has settled flat in a photo.
- Use the external figure. Every airline limit below is external, because the constraint is a fixed box of air under the seat in front of you. A roomy interior is irrelevant to whether the bag goes in.
A carrier that measures over on one axis is over. Nothing averages out.
These are the figures each airline published on its own pet page on August 2, 2026, in inches, with the length, width and height in the order that airline prints them. Eight published policies covering nine airlines, since Alaska and Hawaiian share one pet page. They change, and several of these airlines say in the same breath that the real limit belongs to the aircraft, so treat the table as a starting point and confirm on the airline’s page before you fly. Each source is linked at the bottom of this page.
| Airline |
Soft-sided maximum |
Hard-sided maximum |
Weight rule on the page |
| Alaska and Hawaiian |
17 L x 11 W x 9.5 H |
Not allowed in the cabin |
None published |
| Allegiant |
18 L x 12 W x 11 H |
17 L x 11 W x 10.5 H |
None published |
| American |
18 L x 11 W x 11 H, all flights |
19 x 13 x 9 mainline; 16 x 12 x 8 American Eagle |
20 lb pet plus carrier, Premium Cabin on the three-cabin A321T only |
| Delta |
18 x 11 x 11 recommended as fitting most aircraft, not published as a maximum |
No separate figure published |
None published |
| Frontier |
18 L x 14 W x 11 H |
18 L x 14 W x 8 H |
None published |
| JetBlue |
17 L x 12.5 W x 8.5 H |
Same figure; both types accepted |
None published |
| Southwest |
18 L x 10.5 W x 10 H for a non-Southwest carrier |
No hard-sided figure; the page states one size for any non-Southwest carrier and recommends soft-sided |
None published |
| United |
11 x 18 x 11, axes not labeled |
9 x 17.5 x 12, axes not labeled |
Page states there are none |
Two rows in that table do most of the damage.
Alaska and Hawaiian do not accept a hard-sided kennel in the cabin at all. Their shared pet page says, flatly, “Hard-sided kennels are not allowed in the cabin.” Four of the other seven rows publish a distinct hard-sided figure and JetBlue applies its single figure to both types, so a rigid carrier that is inside the published limit on most of this list still cannot board those two. The same page adds that a pet that does not fit comfortably in its carrier or under the seat “must be placed in a hard sided kennel” and checked into the baggage compartment at the owner’s expense, which is a different trip for the animal than the one you booked.
The published heights are nowhere near each other. JetBlue publishes 8.5 inches. Alaska and Hawaiian publish 9.5. Southwest publishes 10. American, United, Frontier and Allegiant publish 11. That is a 2.5-inch spread on the single axis a soft carrier is most likely to be over on.
Take the tightest number on each axis across all eight (reading United’s 18 as its length, the only reading consistent with an under-seat bag, and treating Delta’s recommendation as its figure) and you get a carrier measuring 17 inches long, 10.5 inches wide and 8.5 inches high. That shape is inside every one of the eight published limits at once. It is our arithmetic on those eight published limits, not a standard anyone issues, and no airline in the table sells or recommends it. It exists to show you the size of the problem, not to send you shopping.
Step 3: The weight rule is not the one most people expect
This is the part that surprised us, and it inverts the advice you usually see.
Of the eight policies checked, exactly one publishes a combined pet-plus-carrier weight cap, and it applies to one cabin on one aircraft. American’s carry-on pet limits table sets it inside the Premium Cabin cell of the row for the three-cabin Airbus A321T: “The total weight of your pet and its carrier must not exceed 20 pounds (9.07 kilograms). We will weigh them at check-in.” That sentence lives in that cell. The Main Cabin cell for the same aircraft carries no weight rule at all, only a cap on how many carriers ride across all cabins. It is not printed as a general American rule, and no other airline we checked publishes anything like it.
United states the opposite position outright: “While there are no weight or breed limitations for pets, they must travel in either a hard-sided or soft-sided carrier.”
Delta, Southwest, Alaska, Hawaiian, JetBlue, Frontier and Allegiant published no pet weight figure at all on the pages we read. For JetBlue we also read the Contract of Carriage (revision 98, dated June 27, 2025) and found no in-cabin pet weight figure there either.
So what actually caps the weight? Two things, neither of them an airline number:
- The carrier manufacturer’s own rating. That is a real limit with real consequences, and it is the figure in each spec table above: 16 lbs for the Sherpa Medium and 22 for the Large, 18 for the Diggs and the Away, 12 for the Sleepypod Atom. Exceed it and the failure mode is a zipper or a seam, not a denied boarding.
- A behavioral test, and the eight pages do not word it the same way. Four of them set a stand-up requirement. Allegiant’s page says pets must “have the ability to stand up and turn around.” Frontier’s says the carrier “Must be large enough for your pet to stand, turn around, and lie down naturally.” JetBlue’s says “Your pet must be able to stand up and move around inside the carrier with ease”, and separately that a pet must “be able to turn around comfortably” once the carrier is closed. Southwest’s says the animal “must be completely inside the pet carrier and be able to stand up and move around the carrier with ease.” The other four set a fit test instead: American, “Pets must be small enough to fit comfortably inside the closed / zipped carrier”; Delta, “small enough to fit comfortably in a kennel with the ability to move around without touching or sticking out from the sides”; Alaska and Hawaiian, “Pets must be small enough to fit and stay comfortably in the carrier under the seat”; United, “This carrier must be ventilated and fit under the seat in front of you or you cannot fly with your pet.” Under either wording, a 19-pound cat in a carrier rated for 18 is not turned away on arithmetic. It is turned away because a person at the gate looks at it and decides it does not fit.
Do not read a missing weight limit as permission to fly a large animal in the cabin. Read it as the airline moving the test from a scale to a counter agent’s judgment, which is worse for you, not better, because you cannot check it in advance.
Step 4: What the soft-sided allowance actually buys
“Soft-sided compresses, so you get extra room” is half true, and these airlines express it four different ways.
Frontier is the only one that publishes the allowance as a number. Its hard-sided maximum is 18 by 14 by 8 inches and its soft-sided maximum is 18 by 14 by 11. Soft buys exactly three inches of height there, and nothing on the other two axes.
American publishes it as a judgment call. Its page says soft-sided collapsible kennels “are recommended and can be slightly larger but still need to fit under the seat without having to excessively collapse the kennel.” Non-collapsible kennels get no such latitude and are held to the under-seat dimensions of every aircraft on your itinerary. Slightly and excessively are the load-bearing words, and neither has a number behind it.
Allegiant closes the loophole with wording instead of a figure. Both of its maximums apply to a carrier that is “fully closed to contain the pet”, so a bag that meets the limit only while squashed does not meet the limit.
Southwest and Delta recommend soft-sided without granting it extra room at all. Southwest’s stated reason is the one to carry with you: “We recommend using a soft-sided carrier, as under-seat space can vary depending on your seat type and location on the aircraft.” That is also why a pass in this checker is not a boarding guarantee. Southwest adds that space for pet carriers “may be more limited in certain seat types, including aisle seats.” Allegiant goes further and prohibits pet carriers in aisle seats on Boeing aircraft outright, citing foot-space limitations.
The practical version: a soft carrier half an inch over on height may board or may not, depending on the airline, the aircraft and the seat you were assigned. A hard carrier that is over is simply over.
The Four Carriers on This Page, Run Through the Checker
Each row below compares the carrier’s published external dimensions against the eight airline figures in the table above, as those figures stood on August 2, 2026.
| Carrier |
Published external size (in) |
Clears |
Over the published figure at |
| Sherpa Original Deluxe (Medium) |
17 x 11 x 10.5 |
American, United, Frontier, Allegiant, and Delta’s recommended size |
Southwest (width and height), Alaska and Hawaiian (height), JetBlue (height) |
| Diggs Passenger |
20 x 10.8 x 11.5 |
None of the eight |
All eight, on length |
| Away Pet Travel Carrier |
18.7 x 10.8 x 10.75 |
None of the eight |
All eight, on length |
| Sleepypod Atom |
17 x 8.5 x 10.5 |
American, United, Frontier, Allegiant, and Delta’s recommended size |
Southwest, Alaska and Hawaiian, JetBlue (height at all three) |
Read the bottom line before the rows. None of the four clears all eight published limits, and height is why. Every one of them stands at least 10.5 inches tall, which is over Southwest’s 10, over Alaska’s and Hawaiian’s 9.5, and well over JetBlue’s 8.5.
The two crash-tested carriers fail differently and harder. The Diggs Passenger at 20 inches external and the Away at 18.7 are over the published length everywhere, including the 18-inch figure that American, Delta, United, Southwest, Frontier and Allegiant all share. That is the cost of the crash rating: both are built around enough rigid internal structure to survive a collision test, and length is where that structure goes.
None of that makes them bad carriers. It makes them car carriers that their makers also market as carry-on compatible, which is precisely the distinction this page exists to draw. If your evacuation is a drive, that is the right trade and the crash rating is the number that matters. If it ends at a gate, measure first and call the airline, because the crash rating and the boarding decision are made by two different parties and only one of them is standing at the airport.
Still choosing a size for the animal rather than the airline? Our pet carrier fit finder takes your pet’s weight and points you at the right category before you compare specific models.
One last caution before you plan around any single figure, including ours. An airline can publish two different answers to the same question in two different places. JetBlue’s pet page, read on August 2, 2026, says this: “A maximum of two pets per traveler is allowed, each in their own carrier.” JetBlue’s own Contract of Carriage, revision 98 dated June 27, 2025, says this: “A Passenger may not transport more than one pet per flight.” Both documents are JetBlue’s. The contract is the binding one, and it is not the one a reader finds first. Confirm a multi-pet fly-out by phone rather than by web page.
The Crash-Test Gap: “Airline Approved” Is Not “Crash Tested”
Here is the fact worth internalizing before you buy: fitting under a seat and surviving a collision are two different tests, and passing one tells you nothing about the other.
Most evacuations are a drive first. You drive to the airport, to a shelter, to a boarding facility, or to a relative’s house, and only some of those trips end with a flight. For the part of the trip that happens in a moving vehicle, the airline label is irrelevant; what matters is whether the carrier was crash-tested, and by whom. The Center for Pet Safety is the only independent US nonprofit that publishes that testing, and its ratings are narrow: a rating covers one exact size, one test weight, and sometimes one specific set of anchor hardware. A 5-star rating on an 18 lb carrier says nothing about a heavier pet crammed into it.
In this roundup, the Diggs Passenger, Away carrier, and Sleepypod Atom carry CPS 5-star ratings; the Sherpa does not. If crash protection on the drive matters to you (and for most evacuations it should), start from the crash-tested three, then confirm the airline fit second. For the full crash-test picture, including rigid crates for larger dogs and how to anchor a carrier in the car, see our vehicle loading and restraints for multiple pets guide, and for the cat-specific crash-tested case, our best cat evacuation carriers roundup.
Flying Out of a Disaster: The Constraints People Don’t Plan For
A fly-out plan feels reassuring until you hit the parts nobody mentions. Planned pet travel is booked weeks ahead. An evacuation is not, and the airline rules do not bend for your emergency.
- Per-cabin pet caps. Airlines limit how many pets ride in each cabin on a given flight. That cap can already be full when you call, and it means a multi-pet household may not be able to fly all its animals together even with a carrier for each.
- Advance reservations. A pet in the cabin usually has to be booked as its own reservation, not added at the gate. Same-day space is not guaranteed, and during a regional disaster, when everyone is trying to leave at once, it is even less so.
- Health documentation. AVMA’s pet-travel guidance points owners to confirm requirements with the airline directly, and many airlines want a recent veterinary health certificate. Getting one the day a hurricane is named is not realistic, so the paperwork has to exist before the emergency, not during it. Our pet travel document organizer gives the health certificate, rabies record, and airline confirmation one fill-in home.
- Shelters are not a fallback for pets. Ready.gov notes that public emergency shelters are only guaranteed to admit service animals, not pets. So the destination has to be a pet-friendly hotel, a boarding facility, or a host arranged in advance, whether you drive or fly.
None of this means flying is off the table. It means air travel should be one branch of a plan that also has a drive-out route and a confirmed pet-friendly destination, not the only branch. For most short-notice evacuations, driving with a secured, crash-tested carrier is the more reliable path, which is why the crash-tested carriers here earn their place even on a page about flying. If a longer relocation rather than a fast evacuation is the situation, our moving long distance with pets guide covers the interstate health-certificate timing, microchip-address updates, and per-species transport rules that a same-day evacuation skips.
Acclimation: The Carrier Can’t Be New on Departure Day
A carrier your pet has never been inside is a carrier your pet will fight, and an airport is the worst possible place to discover that. AVMA and the Red Cross both stress introducing the carrier well before it is needed, through short, low-pressure sessions rather than one forced loading during a crisis.
For a cat especially, that means food-driven acclimation: meals near the carrier, then inside it, then a few minutes with the door closed, worked up over unhurried sessions. Our best cat evacuation carriers roundup walks through the full ASPCA protocol. For any animal, the point is the same: the first time in the carrier cannot be the day you are trying to make a flight.
Heat, Stress, and When to Call the Vet
Travel days are physically hard on animals, and an evacuation stacks heat, noise, and unfamiliar handling on top. This page covers gear and logistics, not diagnosis. If your pet shows heavy panting, drooling, disorientation, vomiting, collapse, or a change in gum color during transport or heat exposure, that is a call for a veterinarian right away, not a home remedy or a “wait and see.”
Flat-faced breeds start that day with less margin, and an under-seat carrier is the least ventilated space they will occupy all trip. Our guide to carrier ventilation for a flat-faced dog covers what real airflow looks like, multi-panel mesh and cross-flow rather than one screened door, and the warning signs that mean you stop and open it.
And never leave a pet unattended in a parked car during evacuation staging, at any temperature. A parked vehicle’s interior heats fast, and a carrier does nothing to slow that. If you suspect your pet ingested something toxic in the chaos of leaving, the ASPCA Animal Poison Control Center is reachable 24 hours a day at (888) 426-4435; a consultation fee may apply.
Before You Buy: Prices, Fit, and the Amazon Note
Every product spec above was pulled from each brand’s own product page, a cited retailer listing, or the Center for Pet Safety’s published test pages: the Sherpa specs were re-checked against Sherpa’s own product page on August 2, 2026, and the rest were pulled on July 9, 2026. The airline figures in the fit checker were read from each airline’s own pet page on August 2, 2026. Prices and airline policies move, and the airline figures move fastest of the three; confirm the current listing and your airline’s under-seat limit before you buy. We don’t display exact prices here: Amazon’s Operating Agreement bars static price display, so we use budget, mid, and premium tiers instead.
Two buying notes worth repeating. The Away carrier is not sold on Amazon (Away’s storefront there covers luggage, not this carrier), so buy it direct from awaytravel.com; the Amazon button will surface other brands. And for any carrier here, measure the external length against your specific airline’s under-seat maximum before you count on it in the cabin, because the airline fit and the crash rating are two separate approvals and only one of them is printed on the box.
Still deciding? For a small pet where cost is the constraint and you mostly need the under-seat fit, start with the Sherpa (the Medium linked here fits pets up to 16 lbs; size up to the Large for up to 22 lbs), and check its 10.5in height against your airline first, because that is the axis it loses on. If the drive is the real leg and the flight is a maybe, the Diggs and the Away carrier are the crash-rated picks, but run the checker above before you plan on either one boarding: at 20in and 18.7in external they were over the published in-cabin length at all eight policies we read on August 2, 2026. The Sleepypod Atom is the one carrier here that is both crash-rated and inside a majority of those published limits, for a pet under 12 lbs.
For the broader evacuation-kit picture, see our pet evacuation kits pillar guide. For the crash-tested car and cargo side of the decision, read vehicle loading and restraints for multiple pets and, for cats specifically, best cat evacuation carriers. Multi-pet households flying or driving out together should also read evacuating multiple cats, and anyone planning a longer relocation rather than a fast evacuation should see moving long distance with pets for the interstate paperwork and per-species rules this page doesn’t cover.