What is the maximum size to fit under a plane seat?

Okay, so you're wondering about the maximum size bag you can squeeze under the seat in front of you, right? I've totally been there, crammed in a middle seat, hoping my bag isn't too big! It's usually around 18 x 14 x 8 inches (length x width x height). Remember that - 18 x 14 x 8. It's a good rule of thumb. Though, you know, sometimes I feel like those dimensions are…aspirational? Like, really? My little duffel bag that's supposedly those dimensions barely fits sometimes. I remember one flight, coming back from visiting my parents, I was sure my bag was the right size, but the flight attendant made me put it in the overhead bin anyway! So frustrating! I think it depends on the specific airline, and even the specific plane. Smaller regional jets? Forget about it. I once had to gate-check a perfectly normal-sized backpack on one of those. Can you believe it? A backpack! So, while 18 x 14 x 8 is generally a good guideline, it's probably smart to double-check with the airline you’re flying just to be safe. Wouldn't want a repeat of my backpack incident!