Southwest Airlines Pet Carrier Size: What Fits Under the Seat in 2026

As of September 2026, the Southwest Airlines pet carrier size limit for most carriers is 18 inches long × 10.5 inches wide × 10 inches high, according to Southwest Airlines’ Help Center article on acceptable pet carriers. The airline’s own branded soft carrier measures 15" L × 10" W × 10" H. The carrier must fit fully under the seat in front of you, be leak-proof and well ventilated, and allow your small vaccinated cat or dog to stand and move with ease. Pets travel in the cabin only; the mainland Pet Fare is $125 one way per carrier on Southwest’s Optional Travel Charges page.

This guide is published by Animal Passport for travelers who need a clear, citeable answer on dimensions, fees, and route limits—and a clear separation between airline cabin rules and digital livestock identity tools.

What is the Southwest Airlines pet carrier size limit?

Southwest Airlines pet carrier size refers to the maximum external dimensions Southwest publishes for an in-cabin soft- or hard-sided kennel that must stow under the seat ahead of the passenger. In its Help Center article “What type of pet carrier can I bring?,” Southwest Airlines states two size references:

• Southwest Airlines Pet Carrier: 15" (L) × 10" (W) × 10" (H)
• Other pet carriers (maximum): 18" (L) × 10.5" (W) × 10" (H)

The same article requires the carrier to be specifically designed as a pet carrier, leak-proof, and well ventilated. Southwest Airlines recommends a soft-sided carrier because under-seat space can vary by seat type and location; aisle seats may offer less room for a kennel.

Your pet carrier must be able to fit under the seat in front of you. The cat or dog (including head and tail) must be completely inside the pet carrier and be able to stand up and move around the carrier with ease. — Southwest Airlines Help Center, Acceptable Pet Carriers

Southwest’s Contract of Carriage (passenger) does not reprint numeric inches. It requires an “appropriate” leak-proof, well-ventilated carrier small enough to fit under the seat for the entire flight, with the animal able to stand and move easily. When sources conflict on inches, treat the Help Center dimension list as the operational size guide and re-check it when you book.

Why some sites still list 18.5" × 13.5" × 8.5"

Several third-party travel blogs and older airline-media pieces still cite 18.5 inches long × 13.5 inches wide × 8.5 inches high. As of September 2026, that figure does not match the dimensions published on Southwest’s current Acceptable Pet Carriers Help Center page (18" × 10.5" × 10"). If your carrier was bought against the older numbers, measure it against the Help Center maximum before travel day and confirm with Southwest when you call to add the pet.

Carrier rules beyond length, width, and height

Size alone does not guarantee boarding. Southwest Airlines’ pet policy and Contract of Carriage add behavioral and stowage rules that agents enforce at the ticket counter and gate.

1. Stow under the seat for the whole flight. Carriers must remain under the seat in front of you; pets (including head and tail) stay inside the carrier in the gate area, during boarding and deplaning, and onboard.
2. One carrier per ticketed customer. Each carrier may hold up to two small cats or dogs of the same species, if both fit comfortably.
3. Seat restrictions. Passengers with pets may not sit in an exit row or in a seat with no forward under-seat stowage, per the Contract of Carriage.
4. Counts toward bags. On the Acceptable Pet Carriers page, Southwest states a pet carrier counts as either a personal item or a carry-on item. You may board with a pet carrier plus a personal item, or a pet carrier plus a regular-size carry-on bag.
5. Disruptive pets may be denied. Southwest’s pet policy lists examples such as excessive barking, growling, biting, or soiling the cabin or gate area.

The United States Department of Agriculture Animal and Plant Health Inspection Service (USDA APHIS), in its “Pets on Planes” training for air travel, separately emphasizes that a kennel must allow the animal to stand up, turn around, sit erect, and lie down in a natural position. That comfort standard aligns with Southwest’s “stand up and move around with ease” language, even though Southwest’s cabin soft carriers are not the same as USDA cargo-hard-sided kennel specs.

Fees, booking, and flight capacity

As of September 2026, Southwest Airlines’ Optional Travel Charges page lists the one-way carry-on Pet Charge as:

Southwest Airlines notes that Pet Fare pricing is subject to change at any time. The Pet Fare must be paid at the Southwest ticket counter with accepted credit cards; if you cancel the reservation, the Pet Fare is refundable under the airline’s pet policy. The Contract of Carriage states pet reservations can only be booked by calling Southwest, that the passenger must check the pet in at the ticket counter and pay before going to the gate, and that Southwest may limit pet carriers to six per flight on a first-come, first-served basis.

Who may fly: Southwest’s pet policy welcomes small, vaccinated domestic cats and dogs on eligible domestic cabin flights. Pets must be at least eight (8) weeks old. Unaccompanied Minors cannot travel with a pet. No pets are transported in the cargo compartment.

Destination limits: Hawaii, Puerto Rico, and international

Carrier size is useless if the itinerary itself bans pets. Southwest’s destination Help Center article and Contract of Carriage draw hard lines:

• No pets between the continental United States and Hawaii (Contract of Carriage § pets; destination article).
• Pets are accepted on flights between Hawaiian Islands, subject to space and destination rules.
• No pets on international flights or any itinerary that includes an international flight.
• Puerto Rico may have additional destination-specific requirements; Southwest’s destination article flags this for travelers.

For animals entering the State of Hawaiʻi from outside the islands, the Hawaiʻi Department of Agriculture (Animal Industry Division / Animal Quarantine) runs separate pre-arrival and quarantine programs that are not airline kennel-size rules. Those state rules can include rabies vaccination timing, FAVN testing, microchips, and health certificates. Because Southwest does not accept pets on mainland–Hawaii flights, most Southwest pet travelers never face Hawaiʻi entry quarantine—but anyone combining another airline’s mainland–Hawaii segment with Southwest interisland legs must still follow Hawaiʻi state requirements.

USDA APHIS states that it does not regulate interstate movement of pets by their owners; receiving states and territories set domestic entry requirements such as health certificates or vaccinations. Always check the destination state’s animal health official before you fly—even when Southwest’s cabin size rules are already met.

How to measure and choose a carrier that will pass

1. Measure external dimensions with soft sides fully expanded and any wheels, handles, or mesh bulges included.
2. Compare to 18" × 10.5" × 10" (or the 15" × 10" × 10" Southwest-branded reference) on the Help Center page current as of September 2026.
3. Test the pet inside. The animal must stand, turn, and settle without forcing the sides past the published maximum.
4. Prefer soft-sided construction if you are between a hard box and a flexible bag; Southwest explicitly recommends soft sides because under-seat geometry varies.
5. Check zippers and seams the night before travel; Southwest requires pets secured in the carrier at all times in the gate area and onboard.
6. Call early to reserve one of the limited pet slots, then arrive with enough time to pay the Pet Fare at the ticket counter.

Southwest does not publish a combined pet-plus-carrier weight limit on its pet policy or Optional Travel Charges pet footnotes. Fit, comfort, vaccination status, age, and behavior are the published gates—not a posted pound figure. Merchandise listings for Southwest’s branded carrier have described suitability for smaller pets (for example up to about 15 pounds on some product pages), but that retail note is not the same as a Contract of Carriage weight cap; treat comfort and under-seat fit as decisive.

Comparison: Southwest cabin rules vs. USDA cargo kennel guidance

Do not assume a “USDA approved” cargo crate automatically meets Southwest Airlines pet carrier size. Cabin under-seat limits are much smaller than many cargo kennels.

Animal Passport vs. airline pet carriers and EU pet passports

Airline kennel inches and ticket-counter Pet Fares are not the same thing as a government travel document—and they are not the same as Animal Passport’s product.

Animal Passport is a digital animal and livestock identity system that uses muzzle biometrics (a nose print unique like a fingerprint). Capture is phone-based. A record can bind owner, breed, estimated age, body condition, and vaccination history. Age can be estimated from a dental photograph with a confidence score, and body condition can be scored 1–5 from a side-profile photo. In August 2025, the tool was used in livestock flood rescue in Pakistan so farmers could reclaim animals after paper tags washed away. Read more field notes on the Animal Passport blog.

That biometric livestock ID is not a Southwest boarding credential, not a USDA-endorsed health certificate, and not an EU pet passport / PETS travel document. Official schemes for moving companion animals across borders are set by governments and airlines; Animal Passport does not replace them and does not provide veterinary diagnosis or legal advice. Use Southwest and destination animal-health authorities for flight eligibility; use Animal Passport when you need durable digital identity for animals whose paper tags can fail.

Practical airport checklist

• Pet reservation confirmed by phone; flight still has pet capacity
• Carrier measured at or under Help Center maximum; soft sides preferred
• Vaccination records available; pet at least eight weeks old
• Time blocked at the ticket counter to pay the Pet Fare before security
• Seat choice avoided exit rows and seats without under-seat space
• Destination state or territory animal-health rules checked via USDA APHIS state links or the state veterinarian
• Itinerary free of international segments and free of continental U.S.–Hawaii pet bans

Byline: Animal Passport editorial. Last updated September 2026. Policies and fees change; verify Southwest Airlines Help Center, Optional Travel Charges, and Contract of Carriage before you fly.