Airlines have varying requirements for pet carriers that fit under their seats. It's essential to double-check with the airline you're flying with for the latest guidelines. For instance, JetBlue specifies dimensions of 17" x 12.5" x 8.5", while American Airlines allows soft-sided bags measuring 18” x 11” x 11” and hard-sided kennels at 19” x 13” x 9”. Delta and United follow the 18” x 11” x 11” rule. Remember, your cat should have room to stand up and turn around comfortably.