Travel without your best friend? Not if you can help it. Major airlines allow passengers to bring their dogs on planes, but only on some flights and only if both dog and owner meet certain requirements. Pass those hurdles, and there’s only one left: paying the fee for flying with a dog. It varies by airline, and it’s not cheap.

What Does It Cost to Fly With a Dog?

A dog doesn’t need an airline ticket, per se; the dog won’t be assigned its own seat. But for passengers who want to travel with their pets, airlines do charge pet fees. The fee is separate from your own ticket. Expect to pay anywhere from $100 to $200 each way. A passenger who pays the fee and meets other requirements may then bring a crated dog into the cabin and place the carrier under the seat in front of her.

Each traveler is usually limited to one dog, with a few exceptions. Southwest allows passengers to put two small dogs in a single carrier. It also allows a passenger who needs to travel with a service dog to bring a second small dog in a carrier.

Because airlines allow companion animals only to fly within the cabin and inside carriers, only small dogs can travel this way. Owners of large dogs may ship them in cargo, a service that many major airlines offer. The cost for shipping a dog in cargo depends on the size of the crate and the distance of the trip, so it varies widely – but expect it to cost at least as much as a plane ticket for the same itinerary would cost.

Flying With a Service Dog

The rules are different for passengers traveling with service dogs. For one thing, airlines don’t charge any fees for approved service animals. They also allow these dogs to sit on the floor or on the traveler’s lap. But because pet owners have gamed the system in the past by pretending regular pets were service animals, airlines do require travelers to provide proof that service dogs are medically necessary. A letter from a licensed mental health professional generally suffices. Call the airline directly to talk about its service-animal policies.

Preparing to Fly With a Dog

Airlines limit the number of pet carriers allowed in the cabin on any given flight, so book your ticket and reserve a spot for your pet as soon as possible to avoid the dog being turned away. Note that some airlines don’t accept dogs in the cabin on international flights, on flights to Hawaii, or on any trip that takes longer than 12 hours.

Find out the airline’s maximum size for an under-seat bag before buying a carrier. Alternately, remove any doubt about whether a carrier will fit by buying one directly from the airline; several carriers sell them. Make sure to spend a few days before the flight helping your dog adjust to the carrier, so being enclosed in it during the trip isn’t traumatic. Take the dog for a check-up and request copies of vaccination records from the vet’s office, just in case. Before leaving home, locate the pet relief area at any airports you’ll be passing through and pack plenty of food and treats to keep the dog happy and comfortable during the trip.