Updated 26 August 2026

The airline size guide

Nine U.S. airlines, the exact size each one publishes for the space under the seat, and which of our carriers gets under it. Read from the airlines' own policy pages, not copied from another shop.

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Start with the one thing nobody tells you

There is no such thing as an airline-approved pet carrier. No airline runs a certification programme, inspects carriers, or issues approvals. What airlines do is publish a maximum size for the space under the seat in front of you, and then leave it to a gate agent to decide, on the day, whether your bag goes under that seat.

So the sticker on the box means nothing. The numbers mean everything.

Two measurements are fixed. The third gives one inch.

The space under an airline seat is a fixed rectangle bounded by the seat legs and the seat above. A carrier 19 inches long does not become 18 inches long because you push it. Length and width have to clear the published figure exactly as they are.

Height behaves differently. Soft-sided carriers compress: the top gives, the mesh flexes, the frame folds. That is not a trick — it is the reason every single airline in the table below recommends a soft-sided carrier rather than a rigid one. But compression has a limit, so ours is one inch. A carrier one inch over the published height goes under the seat; a carrier three inches over does not, and calling that a fit would be doing you no favours.

That single rule is why JetBlue, which publishes the lowest height on the list at 8.5 inches, rules out most of the soft carriers on the market — ours included, with one exception.

The practical test a gate agent applies is simpler than any table: can the bag go under the seat, and can your pet stand up and turn around inside it? Buy for those two questions.

How to measure your pet in two minutes

  • Length. With your pet lying down, measure nose to the base of the tail — not the tip. Add 2 inches. That is the interior length you need.
  • Height. With your pet standing, measure floor to the top of the head.
  • Weight. Weigh yourself, then weigh yourself holding your pet. Some airlines count the carrier in the total.

Between two sizes, go up. Every strap adjusts down; no bag adjusts up.

Before you fly

  • Book the pet the moment you book yourself. Cabins have a fixed number of pet spaces per flight and they go early.
  • Call and confirm the aircraft. Regional jets often have a smaller under-seat space than the mainline aircraft the policy page describes.
  • Practise at home. Two weeks of the carrier being open on the floor with a blanket in it turns a fight into a nap.
  • Plan security. The carrier goes through the X-ray machine; your pet does not. You will carry them through the metal detector in your arms. Put a harness on them before you leave the house.
The numbers

What nine U.S. airlines publish

Every figure below was read on the airline's own policy page. Click an airline name for its own page: the fee, how to book the pet, the age minimum, what it does to your other bags, and which of our carriers gets under that seat. Dimensions are in inches, given as length × width × height.

On weight. Seven of these nine publish no weight limit at all. United states explicitly that there is none. Breeze caps it at 25 lb including the carrier. Everywhere else the gate check is the same two things: the bag fits the space under the seat, and your pet can stand up and turn around inside it.

Airline Soft-sided max (in) Hard-sided max (in) Worth knowing
American Airlines 18 × 11 × 11 19 × 13 × 9 in · American Eagle 16 × 12 × 8 in American Eagle regional jets have a smaller limit than the mainline fleet.
Delta 18 × 11 × 11 No dimensions published Delta publishes 18 × 11 × 11 as a recommendation, not a hard rule: the bag has to fit under the seat in front of you.
United 18 × 11 × 11 17.5 × 12 × 9 in Publishes a separate, slightly shallower limit for hard-sided carriers.
Alaska / Hawaiian 17 × 11 × 9.5 17 × 11 × 7.5 in Since the merger, Hawaiian no longer has its own policy — it shares Alaska's page. Hard-sided carriers are accepted, but two inches shallower than soft.
JetBlue 17 × 12.5 × 8.5 Same as soft-sided The lowest published height on the list at 8.5 inches. Wide, but very shallow.
Southwest 18.5 × 13.5 × 8.5 Same as soft-sided The widest footprint of the majors, but the shallowest ceiling on this list at 8.5 inches. Wide and flat, not tall.
Frontier 18 × 14 × 11 18 × 14 × 8 in The most generous footprint published by any airline on this list.
Allegiant 18 × 12 × 11 17 × 11 × 10.5 in Roomier than the majors, and it publishes a separate hard-sided limit.
Breeze 17 × 8 × 13 Not allowed in the cabin Published as under 17 × 8 × 13. That 8-inch width is narrower than any soft carrier we sell, and the weight limit includes the bag.

Two airlines are missing on purpose. Spirit stopped operating in May 2026, so it is not in the table. Sun Country's policy page blocks automated checks, and we will not publish a number we could not read ourselves.

Policies change without notice, and the airline's page is always the authority — not this one. If you find a figure here that no longer matches the source, write to us and we will fix it the same week.

The tool

Pick your airline. See what actually fits.

Nine U.S. airlines, nine different under-seat limits. Length and width are fixed by the space itself, so a carrier has to clear those as it is. Height is the one figure with any give, and we allow one inch of it — no more.

Soft-sided max
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Hard-sided max
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Weight limit
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Read on the airline’s own policy page, 26 August 2026. Open the official page

Mr. Peanut's Gold Series Low-Profile — 16 × 10 × 10 in, Up to 12 lb Mr. Peanut's Gold Series Low-Profile 16 × 10 in footprint · 10 in tall · up to 12 lb Checking against the published figures $119.99 Under-Seat Pet Carrier — 17 × 9 × 11.5 in, Up to 15 lb Under-Seat Carrier 17 × 9 in footprint · 11.5 in tall · up to 15 lb Checking against the published figures $36.99 Travel Carrier — 17", Side Entry & Seat-Belt Loop Travel Carrier — 17 in 17 × 10 in footprint · 11.5 in tall · up to 15 lb Checking against the published figures $35.99 Soft Carrier Bag — 17" or 19", Top Access, Up to 15 lb Soft Carrier Bag — 17 in 17 × 9 in footprint · 11.5 in tall · up to 15 lb Checking against the published figures $39.99 Cabin Carrier — 17 × 11 × 11 in, Wipe-Clean Nylon Cabin Carrier — 17 in 17 × 11 in footprint · 11 in tall · up to 11 lb Checking against the published figures $34.99 Mr. Peanut's Gold Series Carrier — 18 × 10.5 × 11 in, Charcoal Ash Mr. Peanut's Gold Series Carrier 18 × 10.5 in footprint · 11 in tall · up to 15 lb Checking against the published figures $109.99 Mr. Peanut's Gold Series Expandable — 18 × 10.5 × 11 in, Deja Blue Mr. Peanut's Gold Series Expandable 18 × 10.5 in footprint · 11 in tall · up to 15 lb Checking against the published figures $119.99 Mr. Peanut's Diamond Series — 18 × 11 × 11 in, Double Expandable Mr. Peanut's Diamond Series 18 × 11 in footprint · 11 in tall · up to 15 lb Checking against the published figures $79.99 Mr. Peanut's Platinum Series — 18 × 10.5 × 11 in, Double Expandable Mr. Peanut's Platinum Series 18 × 10.5 in footprint · 11 in tall · up to 15 lb Checking against the published figures $84.99 Backpack Carrier — 16.5 × 12.5 × 10.5 in, Up to 25 lb Backpack Carrier Backpack 16.5 × 12.5 in footprint · 10.5 in tall, lying on its back · up to 25 lb Checking against the published figures $49.99 Pet Backpack — 17 × 13 × 12 in, Up to 16 lb Pet Backpack Backpack 17 × 13 in footprint · 12 in tall, lying on its back · up to 16 lb Checking against the published figures $59.99

How we count. The footprint has to clear as it is — a bag does not get narrower. On the height we allow one inch of squash, because a soft top gives a little under a seat. One inch, not four: a bag three inches over the published height is not going under anything, and we are not going to pretend otherwise. Confirm the policy with your airline before you book, and remember the rule a gate agent actually applies — your pet must be able to stand up and turn around inside.

The two backpacks are in this list on purpose. They are measured lying on their back, because that is the only way a backpack goes under a seat, and on those figures each one clears exactly one of the nine: Frontier. Everywhere else they do not fit under a seat, so they are not a way to fly with your pet, and the tool will tell you which airline is refusing them and by how much.

Still unsure

Questions this guide gets

Shop under-seat carriers

Not to the millimetre, and that is the airlines' own position: every airline in the table recommends a soft-sided carrier precisely because it compresses into the space. But the give is small. We allow one inch over the published height and count anything beyond that as a miss.

What a gate agent checks is whether the bag goes under the seat and whether your pet can stand up and turn around inside it. A bag an inch proud of the limit passes that test. A bag three inches proud does not.

Then we have not read its policy ourselves, and we will not publish a number we did not verify. Write to us with the airline and we will check it and add it.

Every three months, and immediately whenever a reader tells us a figure no longer matches the source. The date at the top of this page is the last time every row was read on the airline's own site.

Some airlines allow two small animals of the same species in one carrier if they still fit comfortably; others do not. This is the question most worth calling about rather than reading about — the published policies are vague and the agent on the phone is not.

Free, one page

The pre-flight checklist is free, and it is already online

Everything to do in the four weeks before you fly with a pet, in the order you have to do it. Read it now — no email needed. Leave your address and we will send you a 10% code for your first order — plus a warning if your airline moves its published limit before you fly.

  • When to call the airline, and what to say when you do
  • The paperwork each airline asks for, and how far in advance
  • How to get a cat used to a carrier in two weeks
  • What happens at security, step by step