An FAA-approval sticker doesn't guarantee your car seat will fit on your specific airline, said Dr. Baer. Car seats can vary in width, usually from 17 inches to 21 inches. It's best to check your car seat dimensions against the seat dimensions of your airline. If you purchased a ticket for your child, you are entitled to use the child's car seat in his or her airplane seat. 2  If the car seat doesn't fit in your assigned seat, the airline has to find a different seat that will fit the car seat in the same class of service.