The rules come from your airline, not from the label
“Airline approved” is a marketing phrase, not a certification. No agency approves carriers; each airline publishes its own maximum under-seat dimensions, and those dimensions differ between carriers and even between aircraft types on the same carrier. Before you buy anything, find your airline's pet policy page, note the exact under-seat measurement for the plane you are booked on, and shop to that number. A soft-sided carrier with a flexible top can squeeze into a slightly tighter space than its listed dimensions suggest, which is why soft carriers dominate cabin travel.
Cabin or cargo: the decision is mostly about size
Most airlines allow pets under roughly 15 to 20 pounds, carrier included, to fly in the cabin under the seat in front of you. Larger dogs travel as checked baggage or cargo, in a rigid kennel that meets IATA standards: hard plastic shell, metal hardware rather than plastic clips, ventilation on multiple sides, and a spring-loaded door. Book pet spots early — airlines cap the number of animals per flight, and cabin slots for popular routes fill weeks out. Brachycephalic breeds such as pugs and bulldogs face temperature and breed restrictions on many airlines, so check that first if you have one.
Measure the dog, then measure the carrier
The carrier must let your pet stand, turn around and lie down naturally. Measure from the base of the neck to the tail for length, and from the floor to the top of the head or ears while standing for height, then add a few inches to each. Buying too large is the common mistake: an oversized carrier gets rejected at the gate, and a pet actually rides more comfortably with gentle contact on all sides. Look for a padded, removable and washable base, mesh on at least two panels, a leash clip inside, and a strap that slides over a suitcase handle.
Make the carrier a good place weeks in advance
Leave the carrier open in your living room a month before the flight with the door removed and a familiar blanket inside. Feed meals in it. Toss treats in without ceremony. Then build up: a closed door for two minutes, a walk around the block, a short car ride, a coffee-shop trip. A pet that has only ever entered the carrier on the way to something stressful will fight it in the terminal. One that has napped in it for weeks will usually settle once the cabin lights dim.
Airport day, step by step
Exercise hard that morning, then feed a small meal three to four hours before departure and stop water an hour out. At security, you carry the pet through the metal detector while the empty carrier goes through the scanner — practice holding a squirming animal one-handed before you are standing in the line. Attach a slim collar with ID and clip a leash to the harness before you unzip anything, ever, so a startled pet cannot bolt in a terminal. Many large airports now have indoor relief areas past security; find yours on the terminal map in advance.
Skip the sedatives, pack the paperwork
Most veterinarians advise against sedation for flights because altitude affects respiration and blood pressure, and a sedated animal cannot reposition itself if it slides. Ask your vet about calming supplements or a pheromone spray on the carrier bedding instead. Carry a health certificate if your destination requires one, keep vaccination records with your boarding pass, and freeze a small water dish that thaws into drinkable water mid-flight. Land, find grass, walk before you do anything else — the reunion routine matters as much as the departure one.