What do you do if you find your carry on won't fit under the seat or in the overhead of an airplane?

You give it to the flight attendant to check. It should fit if there is available room, oversize luggage doesn’t get through security.

If you check it once on board, you don;t get charged. In fact, on most United flights they ask before boarding if anyone is willing to check their bags for free, especially for those in boarding groups 4 and later.

Most of the time they will tag it and put it in the same places that they put strollers and car seats. You just wait after you get off and they give it back to you.

If it’s larger than a certain size you cannot take it on the plane as a carry on.

If there’s no room in the overhead due to too many bags being in there already, the flight attendant will find room in another overhead to put your bag. If necessary, you would have to check it.

If it’s regulation size but you’ve merely overpacked it enough to make it not fit, a flight attendant, one of those good ones who takes a little extra time, can sometimes figure out a way to get it in the overhead bin by re-arranging other bags and turning yours a certain way. I’ve seen that happen a number of times. But if it just can be made to fit, you simply give it to her to be tagged and stowed below.

On my last trip, many passengers had to make sudden alternate arrangements due to cancelled flights. The flight attendants were tagging our baggage and handing it out the back door to the baggage handlers.

You can check the bag at the gate.

Some airlines have a box-type thing at check in, that you can stick your carry-on into and if it fits, it goes on the plane, and if it doesn’t it has to be checked. Therefore, no matter how “fat” you stuff it, there’s no confusion about what gets carried on and what does not.