United Airlines Pet Carrier Size: What Fits Under the Seat

United Airlines pet carrier size is the cabin kennel limit for a cat or dog. As of September 2026, United Airlines’ traveling-with-pets page, archived December 22, 2025, allows a hard-sided carrier up to 17.5 inches long, 12 inches wide, and 9 inches high, and a soft-sided carrier up to 18 inches long, 11 inches wide, and 11 inches high. On a Boeing 737 MAX, height is capped at 10 inches. The carrier must be ventilated and fit under the seat in front of you.

Those inch figures come from the image descriptions on that United page, not from a reseller chart. This article was last updated in September 2026 by Animal Passport. The live united.com page did not load during this review, so confirm it before you pay the pet fee.

What United Airlines pet carrier size actually controls

United Airlines pet carrier size is an airline acceptance limit, not a federal kennel code for animals riding with their owners. The United States Department of Agriculture Animal and Plant Health Inspection Service (USDA APHIS) says transportation of pets that remain in the owner’s custody is not regulated under the Animal Welfare Act. On a page last modified April 17, 2026, USDA APHIS gives the cabin pet traveling with its owner as the example. Airlines that carry animals under the plane or as cargo are regulated.

United’s archived page says there are no weight or breed limitations, but the animal must travel in a hard-sided or soft-sided carrier. The only animals United says may board without a carrier are service animals. Therapy cats and dogs, and emotional support cats and dogs, must fly as pets and follow the carrier rules at the airport and on the plane.

United says it no longer lets passengers check pets through the PetSafe program. Pets can fly in cargo only when traveling with certain active-duty military or State Department employees. A later answer on the same page describes exemptions for U.S. military members on a permanent change of station and their spouses, and for State Department foreign service officers and their spouses. Those two sentences are not identical. If you think you qualify, ask United which wording applies to your orders. For everyone else, a pet that misses the cabin size has no public cargo backup on United.

Hard-sided and soft-sided maximums

United publishes two outside maximums. A carrier that matches the general soft-sided size can still fail on a Boeing 737 MAX, because that height cap is shorter.

Treat those numbers as outside dimensions. A soft bag sold as 18 by 11 by 11 inches is at United’s ceiling. Wheels, pockets, or a pet pressing the fabric can push it over. United’s traveler note says the carrier must be ventilated and fit under the seat in front of you, or you cannot fly with your pet.

The International Air Transport Association (IATA) adds a welfare test United’s inch caps do not replace. In its Guidance for Passengers Traveling with their Dog or Cat in the Cabin, IATA says the container must fit under the seat in front of the passenger, or in another place the airline specifies. The dog or cat should be able to sit erect, stand erect, turn around, and lie down in a natural position. IATA says it does not certify or endorse any commercial carrier. Individual airlines may be stricter than Container Requirement 1 of the IATA Live Animals Regulations.

IATA also warns that soft-sided containers may compress under the seat, and that compressing the ventilation openings can cut airflow and let heat build up. IATA’s checklist asks whether at least 16 percent of the four vertical sides is ventilation, whether the box is escape-proof and leak-proof, and whether a “Live Animals” label is attached. United’s public size page does not repeat every construction detail.

Why the Boeing 737 MAX changes the height

United’s seating footnote says that on a Boeing 737 MAX the pet carrier can be no more than 10 inches tall. The soft-sided maximum on the same page is 11 inches high. An 11-inch soft carrier that meets the general soft-sided limit is one inch too tall for a 737 MAX before it ever goes under the seat.

The MAX limit overlaps United’s one-pet rule on several narrow-body jets, including the 737-8 (MAX 8) and the 737-9 (MAX 9). Check the aircraft code on the reservation before you buy a carrier. A hard-sided box at 9 inches high is under the 10-inch footnote, but it still has to slide fully under the seat on that airplane. United says some planes have limited space under the middle seat, which is why those fleets allow only one pet per person.

How to measure the animal, not just the bag

IATA’s Traveler’s Pet Corner gives a formula for minimum internal size. It is not United’s outside maximum. The smaller constraint wins. If the inside is too small for the animal, a bag that meets United’s outside numbers is still the wrong bag.

1. Measure A, length from the tip of the nose to the base of the tail.
2. Measure B, height from the ground to the elbow joint.
3. Measure C, width across the shoulders or the widest point, whichever is greater.
4. Measure D, standing height from the top of the head or the ear tip to the floor, whichever is higher.
5. For one animal, IATA’s minimum internal length is A plus half of B, width is C times 2, and height is D plus bedding.
6. IATA says snub-nosed breeds require a container 10 percent larger. If more than one animal is measured, use the largest.

On United, a second pet needs its own seat on flights that allow two pets at all. IATA says unweaned animals and animals under 8 weeks must not travel, and that airlines and countries may set a higher age. IATA discourages sedation for cabin or cargo pets, a position it says veterinary groups such as the American Veterinary Medical Association also hold. If a veterinarian directs a drug for a real medical reason, IATA says the airline should be told and the dose noted on the health certificate.

Fee, booking, and the rule on the airplane

On the December 22, 2025 archive, pet tickets are only for cats or dogs and cost $150 each way. Select “Travel with a pet” while booking on united.com, or add the pet in My Trips. United says you cannot add a pet in the United app. International trips require a phone call to add the pet. If a partner airline is on the ticket, check that carrier’s policy too. United says the pet stays in the cabin with you if space is available.

Check in at the ticket counter with your confirmation number and the pet’s. United says it issues a special bag tag for the carrier. Pet strollers are checked in the lobby and count toward the checked-bag allowance. On board, the pet must stay in the carrier with the door closed, under your seat, at all times. IATA’s cabin guidance matches that: do not remove the animal or open the container during the flight. The passenger is responsible for the animal, and the container should hold any waste, stay leak-proof, and include absorbent bedding.

Which seats and aircraft accept the carrier

United’s archived page says that on most flights you may bring up to two pets per person if you buy two adjacent seats. It then allows only one pet per person on the Airbus A319, Airbus A320, Airbus A321neo, Boeing 737-800 and 737-8 (MAX 8), and Boeing 737-900 and 737-9 (MAX 9). A later section says “while you can only bring one pet with you,” then lists cabin-wide caps. Those passages disagree. Use the rule United applies to your flight number.

United says you cannot sit with a pet in an emergency exit row, a United Premium Plus seat, or the front row of any cabin. On the 737-800, 737-8, 737-900, and 737-9 in Economy, you must be in an aisle or window seat so the carrier fits. United says these seating notes are for United flights and that passengers should ask United about United Express.

Age, rabies papers, and what is not a passport

United’s archived page sets the age floor it will enforce. On domestic flights, puppies and kittens must be at least 2 months old. On international flights, United says dogs must be at least 6 months old to comply with Centers for Disease Control and Prevention (CDC) requirements, and kittens must be 4 months old to meet vaccination requirements.

The CDC’s media statement of May 8, 2024, is the primary source for dogs entering the United States. Starting August 1, 2024, CDC said every dog entering the country must appear healthy, be at least six months old, be microchipped, and travel with a CDC Dog Import Form receipt. Further rules depend on where the dog has been in the past six months and whether it was vaccinated in the United States. Dogs from countries with a high risk of dog rabies must be protected against rabies. CDC said the rule moves closer to World Organisation for Animal Health standards for dogs from high-risk countries.

United requires rabies and health certificates for dogs and cats on international flights, and says rabies vaccination must be completed at least 28 days before travel, both to and from the United States. It tells passengers to check CDC country rules at least two months ahead. Inside the United States, United encourages a veterinarian’s health certificate listing your name and contact details, the pet’s breed, sex, age, color, and markings, and recent vaccines with dates. Some arrival points may ask for it.

USDA APHIS, on a page last modified June 10, 2026, says it does not regulate interstate movement of pets by their owners. The destination state or territory sets those rules. APHIS does regulate businesses that transport pets for owners. For a pet leaving the United States, APHIS says to contact a USDA-accredited veterinarian early, because the destination country writes the entry rules. APHIS pet-travel guidance last modified March 20, 2026, also points dog importers to the CDC, including a Certification of U.S.-Issued Rabies Vaccination form for certain U.S.-vaccinated dogs that were in a high-risk country.

None of those papers is a carrier, and none of them is the digital record from Animal Passport. European Union pet passports and national pet-travel schemes are official animal-health documents. Animal Passport is a phone-based livestock identity record that uses muzzle biometrics: a nose print unique in the way a fingerprint is unique. The record can bind the owner, breed, an estimated age, body condition, and vaccination history. Age comes from a dental photograph with a confidence score. Body condition is scored from 1 to 5 from a side-profile photo. During livestock flood rescue in Pakistan in August 2025, it helped farmers reclaim animals after paper tags washed away. It is not a veterinary diagnosis, not legal advice, and not a government travel document United, the CDC, or USDA APHIS will accept instead of their forms. Related reporting is on the Animal Passport blog.

Routes where the carrier is not enough

A legal United Airlines pet carrier size does not open a banned route. The December 22, 2025 archive says United does not allow pets to, from, or through Australia, Barbados, Brazil, Cuba, Dominica, French Polynesia (Tahiti), Greenland, Guam, Guyana, Hawaii, Hong Kong, Iceland, Ireland, Jamaica, the Marshall Islands, the Federated States of Micronesia, Mongolia, Morocco, New Zealand, the Northern Mariana Islands (Saipan), Palau, the Philippines, Saint Kitts and Nevis, South Africa, Spain, Sweden, Taiwan, Thailand, Trinidad and Tobago, the United Arab Emirates (Dubai), the United Kingdom, and Vietnam.

The footnotes narrow three of those names. United allows a pet from Guam to Tokyo-Narita or to the U.S. mainland, but not back to Guam, and treats Guam-to-mainland travel as international. The Spain bar is to and from Bilbao only. The Taiwan bar is Kaohsiung to and from Tokyo-Narita, and Taipei to and from Guam. Other countries can still refuse an animal United would carry.

Check the fit before you leave home

1. Write down the aircraft type, especially any Boeing 737 MAX, and whether the seat is a window, aisle, middle, exit, Premium Plus, or front-row seat.
2. Measure outside length, width, and height, including pockets and wheels. Hard-sided maximum: 17.5 by 12 by 9 inches. Soft-sided maximum: 18 by 11 by 11 inches. On a 737 MAX, reject anything taller than 10 inches.
3. Measure the pet with IATA’s A, B, C, and D method. The animal should still sit, stand, turn, and lie down inside the box.
4. Add the pet on united.com or in My Trips for a domestic trip, or call for an international trip. The archived fee is $150 each way.
5. For a dog entering the United States, follow the CDC Dog Import Form, microchip, and six-month rule in force since August 1, 2024. For export, start with a USDA-accredited veterinarian. For another U.S. state, ask that state’s animal health official.
6. Check in at the ticket counter with both confirmation numbers, and keep the door closed under the seat for the whole flight.

As of September 2026, the United Airlines pet carrier size on the December 22, 2025 archive is 17.5 by 12 by 9 inches hard-sided and 18 by 11 by 11 inches soft-sided, with a 10-inch height cap on the Boeing 737 MAX, and the box still has to fit under the seat.