I would be more concerned about my hips fitting between the armrests, then whether or not I need an extension.
The airline will provide you with an extension for each flight. And there is no shame in using one, but if you are worried about the appearance, you can wear a loose fitting, un-tucked blouse that will cover over the fact you are using the extension while seated.
But the width of the seat is the biggest factor in terms of enjoying your flight.
Don’t worry. For example, check http://www.thinnertimesforum.com/topic/54606-q-to-those-that-dont-need-the-seatbelt-extension-in-airplanes/
I did not need to wear an extension when I flew last time – I am 5’9" and weighed 260ish pounds. I was in an 18-20 size clothes.
last time I flew, I was prob around 300 lbs. (or a bit higher) and I didn’t use an extender. Never have actually, and my highest was 330lbs. Size 22 or 24 pants at lane bryant and I’m 5"6.
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