Short answer: Two checks decide whether your pet can fly in the cabin. First, the carrier's outside dimensions must fit your airline's under-seat limit (American: 18 × 11 × 11 in for soft-sided). Second, your pet must be able to stand, turn around and lie down inside it. Measure your pet, then use the calculator to check both.
Key takeaways
- American Airlines publishes 18 × 11 × 11 in for soft-sided carriers, 19 × 13 × 9 in for hard-sided on mainline and 16 × 12 × 8 in on American Eagle.
- Delta sets the maximum by aircraft and recommends a soft-sided kennel of up to 18 × 11 × 11 in, which fits most of its aircraft.
- Height is the usual deal-breaker. Many backpack carriers are taller than 11 inches.
- At security, TSA asks you to take your pet out of the carrier and carry it, or walk it on a leash, through the metal detector.
- Airline rules change. Check the airline's own pet page before you book, and again before you fly.
Shop this guide
- Expandable soft carrier: its 18 × 11 × 11 in option sits exactly at American's soft-sided limit.
- Ventilated carrier backpack: mesh panels and an inside safety leash, for car trips and vet visits.
- PetAmi carrier backpack: for walks and car rides.
- Pet Lodge plush fleece bed: fits inside crates and carriers, per its listing, and is machine washable.
Zavyro tool
Carrier fit calculator
Enter your pet's measurements and a carrier's outside dimensions, or pick one of ours. Choose the airline limit to check against. The calculator checks the carrier against the limit, and your pet against the carrier.
Airline figures verified on the airlines' own pages on 23 September 2026. Always confirm with your airline before flying.
How to measure your pet for a carrier
From the tip of the nose to the base of the tail (not the tip), with your pet standing naturally.
From the floor to the top of the head while standing. For dogs with upright ears, measure to the ear tips.
Weigh yourself holding your pet, then yourself alone, and subtract. Some cabins limit pet plus carrier weight.
Use the listed outside dimensions to check the airline limit, and measure the inside to check your pet.
American Airlines requires that pets fit comfortably inside the closed carrier and can stand, turn, sit and lie down in a natural position. The calculator turns that into three geometric checks: standing height against carrier height, body length against carrier length, and body length against the floor diagonal for turning.
Airline under-seat limits we verified
| Airline | What the airline publishes | Notes |
|---|---|---|
| American Airlines | Soft-sided: 18 × 11 × 11 in (46 × 28 × 28 cm), all flights. Hard-sided: 19 × 13 × 9 in on mainline, 16 × 12 × 8 in on American Eagle | The pet must fit comfortably in the closed carrier and be able to stand, turn, sit and lie down. On the A321T premium cabin, pet plus carrier must not exceed 20 lb. You may bring one additional personal item or carry-on. |
| Delta | Maximum depends on the aircraft; Delta recommends a soft-sided kennel of up to 18 × 11 × 11 in, which fits most aircraft | Must fit fully under the seat in front; leak-proof, with ventilation on 3 sides for domestic travel. One pet per kennel, with limited exceptions. Pets must be at least 8 weeks old for domestic travel. |
| Southwest | Carrier stowed under the seat. Dimensions: check Southwest's page | Each carrier may hold up to two small cats or dogs of the same species. One pet carrier per ticketed customer. Pets must be at least 8 weeks old. |
For any other airline, choose "Other airline" in the calculator and type in the limit from its pet page. Limits change. Check again before you fly, especially if your flight or aircraft changes.
Our carriers against American's soft-sided limit
| Carrier (listed outside size) | Within 18 × 11 × 11 in? |
|---|---|
| Expandable soft carrier, 18 × 11 × 11 in option | Yes, exactly at the limit |
| Expandable soft carrier, 17 × 11 × 9.5 in option | Yes |
| Expandable soft carrier, 18 × 13.5 × 9.5 in option | No: 13.5 in wide |
| PetAmi carrier backpack, 16.5 × 12.5 × 12 in | No: 12.5 in wide and 12 in tall |
| Ventilated carrier backpack, 12.8 × 11.6 × 16.5 in | No: 16.5 in tall |
| Bubble backpack, 12.6 × 11.4 × 16.5 in | No: 16.5 in tall |
Backpacks are great for walks, car trips and vet visits, but most are too tall for an 11-inch under-seat limit, whatever the label says. For flying, check the numbers, not the words "airline approved."
At the airport: what TSA asks
- Take your pet out of the carrier just before screening starts.
- Carry your pet through the walk-through metal detector. Or walk them through on a leash; a well-fitted harness is more secure than a collar. See the Dog Harness Size Calculator.
- Put the empty carrier on the conveyor belt to be X-rayed. Pets never go through the X-ray machine.
- If your pet may bolt, ask a TSA officer to screen you in a private screening room.
Worked examples
Cat, 16 in long, 10 in tall, expandable carrier 18 × 11 × 11, American soft-sided
Carrier within the limit on all three sides. Stand: 10 of 11 in, tight. Lie: 16 of 18 in, pass. Turn: 16 in vs a 21.1 in floor diagonal, pass. Fits.
Small dog, 19 in long, 12 in tall, same carrier
The carrier passes the limit, but the dog is taller than 11 in and longer than 18 in. Won't stand or stretch out, and no carrier within an 11-inch height limit can fix that. Ask your airline about other options, or drive.
Cat in the ventilated backpack, American soft-sided
The backpack is 16.5 in tall against an 11 in limit. Over the limit, however well the cat fits inside.
Cat, 15 in long, 9 in tall, 17 × 11 × 9.5 carrier, Delta
Within Delta's recommended size. Stand: 9 of 9.5 in, tight. Lie: 15 of 17 in, pass. Turn: 15 in vs 20.2 in, pass. Fits, but measure the inside height.
Getting your pet used to the carrier
- Start weeks ahead. Leave the carrier open at home with a familiar blanket and treats inside.
- Practice short car rides in the carrier before the trip.
- Practice the TSA step: taking your pet out of the carrier, and putting a harness on, in a calm room first.
- For older pets, plan extra rest breaks. Traveling With an Older Dog covers planning for seniors.
Holiday travel: what the AVMA adds
- Paperwork. The AVMA notes that interstate and international travel regulations require a health certificate for any pet you bring with you. Ask your veterinarian well before the trip, and check your airline's own requirements.
- Pack for your pet. Bring medical records and identification information, as the AVMA suggests, along with food and any medications.
- Driving instead? The AVMA says pets in vehicles should always be safely restrained, and never carried in the bed of a truck. A carrier that fits your pet works in the car, too.
- Current ID. Airports and relatives' homes are unfamiliar places. Check the tag and the microchip contact details before you leave.
Common mistakes
- Trusting "airline approved" without checking the numbers against your airline.
- Measuring to the tip of the tail. Airlines and carrier makers use body length to the base of the tail.
- Forgetting the carrier counts as a bag. American and Delta allow one additional personal item or carry-on with a pet.
- Checking rules only once. Aircraft changes can change the under-seat space; Delta says so directly.
When a cabin carrier isn't the answer
- Pets taller or longer than a compliant carrier can't travel in the cabin comfortably. Ask the airline about other options, or drive.
- Anxious or unwell pets: ask your veterinarian whether flying is a good idea for this trip.
- Two pets: Southwest allows two small pets of the same species in one carrier; Delta generally allows one per kennel. Check your airline.
FAQ
What size pet carrier fits under an airplane seat?
It depends on the airline and aircraft. American publishes 18 × 11 × 11 in for soft-sided carriers. Delta recommends up to 18 × 11 × 11 in soft-sided, which it says fits most aircraft.
Does my pet have to be able to stand up in the carrier?
American Airlines requires that your pet can stand, turn, sit and lie down in a natural position inside the carrier. Measure your pet's standing height and compare it with the carrier's inside height.
Can a soft carrier be slightly bigger than the limit?
Soft carriers flex, but don't rely on it. Check your airline's rules and choose a carrier within the published dimensions.
Do I take my pet out of the carrier at security?
Yes. TSA asks you to take the pet out just before screening, carry it (or walk it on a leash) through the metal detector, and send the empty carrier through the X-ray machine.
Can two pets share one carrier?
Southwest allows up to two small cats or dogs of the same species per carrier. Delta allows one pet per kennel, with limited exceptions. Check your airline.
Is a pet backpack allowed on a plane?
Only if its dimensions fit the airline's limit. Many backpacks are taller than an 11-inch limit.
Sources and methodology
How the calculator works. It compares the carrier's listed outside dimensions with the airline limit you choose, side by side. It then compares your pet with the carrier: standing height against carrier height, body length against carrier length, and body length against the floor diagonal as a turning check. The turning check and the 1.5-inch "tight" threshold are our geometry, not airline rules.
- American Airlines, Traveling with pets: carrier dimensions, fit requirement, A321T weight limit, carry-on allowance (checked 23 September 2026).
- Delta, Pet travel overview: aircraft-dependent limits, recommended kennel size, ventilation, age, one pet per kennel (checked 23 September 2026).
- Southwest, Pet policy: two pets per carrier, age, under-seat stowage (checked 23 September 2026).
- Transportation Security Administration, TSA tips on traveling with pets through a security checkpoint: screening steps.
- American Veterinary Medical Association, Winter holiday pet safety: health certificates, packing records and ID, restraint in vehicles.
- Zavyro product listings: carrier outside dimensions as published.
Limitations. We verified three airlines' own pages. For other airlines, enter the limit from their page. Listed carrier dimensions are outside measurements from suppliers.
Related guides
- Traveling With an Older Dog: Comfort, Mobility & Planning Guide.
- What to Pack for a Dog Walk: The Printable Checklist. Half of it applies to travel days.
- Raised Dog Cot for Camping, Patio and Travel.
- Dog Bed Size Calculator: the same length measurement, at home.
- Senior Pet Daily Comfort Guide.
- Dog Harness Size Calculator.
- Christmas Gifts for Cats and Cat Lovers.
