The size ceiling
In-cabin travel is for small dogs and cats that fit in a ventilated carrier under the seat in front of you — typically up to 8–9 kg combined with the carrier. Above that, they travel in the hold or as cargo.
Long-haul is different
Several carriers allow in-cabin pets on short and medium routes but not on long-haul international flights. Always confirm the specific flight, not just the airline’s general policy.
Book the pet before the seat
Airlines cap the number of pets per cabin. Reserve your pet’s spot as soon as your route is set — it is a scarcer resource than your own seat.