Just a note. If you need a seat belt extender, you can NOT sit on the exit row. And, having to be moved to another row after the entire plane has boarded is VERY humiliating.

I think it makes sense to go by hip measurements as most airlines are listed on seatguru.com they have the width of the seats listed by plane model. I would suggest booking a flight on a plane with larger seats even a 1/2 inch can make a huge difference. The most uncomfortable flight I had was at close to 300lbs and 57" hips. IT was a small 50ish seat plane with two seats on each side. Luckily it was not a full flight! I would have never been able to fit another person next to me! The flight back was a ok I had a seatmate but the seats were a bit larger. I felt bad as if I was squishing him. I recall saying something to him like I am sorry if I am squishing you and he said no you are fine. I was not in his seat or anything but I was on the armrest wich is pushing it!
Last flight I was around 260lbs and it was so much better I felt big still but definately did not feel I would enroach in anyones space.

Also I have never needed the extender at 300lbs I was at the end of the belt though!

I was on a plane in 2004, a small one from and to Vegas. The seatbelt was very tight, in fact on one flight I almost didn't get it to click. The seat felt uncomfortably small and that awful feeling overcame me. On the flight back, I was next to an obese man, so we were definitely squished together the whole flight which really bothered me. He finally crossed his arms over his belly so I could push my shoulder behind his arm. We didn't speak, he closed his eyes to avoid dealing with any of it. I looked out the window and felt like crying.

I wish I could remember what I weighed, but it was somewhere between 280 and 299. I will be flying to Hawaii in July to watch my 5th grandchild being born, I estimate that I will be about 220 by then, so hoping it is no longer an issue. But wow, there is nothing more humiliating than realizing that one's body is big to fit in normal spaces. :(

Oh I can so completely relate!! I was about 250 flew to NC for my sister's funeral and on the way down I didn't have anyone sitting next to me. I felt like I fit in the seat fine and the seatbelt clicked fine. It was an aisle seat. On the way back I had the window seat and altho I fit fine and my seat belt clicked fine...I was uncomfortable and leaned against the window. I didn't want any of "me" touching the tall, broad (not fat) guy sitting next to me. Flying is always a source of anxiety for me...not the actual flying...but if I will fit in the seat and what will people think of me who have to sit next to me. :o

okay, so i flew recently ... airline had 18'' seat width, accoring to seatguru ... i am about 5'7'', 310 pounds, pear-ish shaped, w really wiggly giggly fat, especially in the thigh area ... seat belt fit no problem ... there was about an inch extra when i secured it tight ..... but i had a ton of thigh over hang on both sides of the seat ... so if you are not traveling with a friend or family member, this will definatly be an issue.... hope this helps :) ... also, i can fit into most size 26 jeans... i sometimes have trouble with skinny jeans that don't provide generous thigh room ....