Whether a duffle bag fits under an airplane seat depends on its dimensions and "squishability." Most major airlines (like Delta, United, and American) have an under-seat "personal item" limit of approximately 18 x 14 x 8 inches. A standard "small" duffle bag (roughly 30–40 liters) will usually fit, provided you do not overpack it to the point where it becomes a rigid cylinder. The benefit of a duffle bag over a hard-shell suitcase is its flexibility; if the bag is only 75% full, you can compress it to fit into the metal sizer at the gate. However, in 2026, budget carriers like Spirit, Frontier, and Ryanair have become increasingly strict, using automated sensors to detect bags that protrude even an inch into the footwell. If your duffle is a "weekender" size (22 inches or longer), it will likely be flagged as a carry-on and required to go in the overhead bin. Always check your specific aircraft type, as regional jets (like the Embraer 175) have significantly smaller under-seat spaces than wide-body jets like the Boeing 787.