What are the standard underseat carrier dimensions for U.S. airlines?

Most major U.S. carriers cap soft-sided pet carriers near 18x11x11 inches, though Alaska Airlines is tighter at 17x11x9.5 inches and Southwest/Frontier sit around 18.5x13.5x9.5. Specialist-subreddit threads on r/delta, r/AlaskaAirlines and r/SouthwestAirlines repeatedly note that soft carriers can squish under most seats, but published dimensions are sometimes enforced at the gate, so buying to the strictest airline you'll fly is the safer play.