Best Answer - Chosen by Voters
How long a flight is this? And is this airline the only one you can fly? I've flow WestJet before and they require a soft carier. The soft carrier I used was about 11" high I just had to squish it in there quite a bit because the clearance was only 8.5" (which I realized a bit too late). My cat wasn't too happy but it was only an hour and half flight.
Now I only fly Alaska which allows hard carriers. The hard carrier I use is just over 9" high and easily slides under the seats of their 737-800 planes. It's also wide enough and long enough that my cat can turn around and even sit up.
If possible I would switch the flight to a different airline that has more space for the carriers. Unless your cat is smaller like mine - she's only about 9 1/2 pounds - then he'd be very uncomfortable in a small carrier squished with only 8.25 inches clearance. Especially if this is a several hour flight.
The other option, of course, is to put your cat in the cargo hold. Then he can be in a larger, much more comfortable hard carrier. I don't recommend if you can avoid it, though. Much more secure knowing your cat is safe right there with you than in the cargo hold where someone might not turn on the air flow, or someone drops the carrier and your cat escapes, etc.
Source(s):
My cat's flown a few times for shows and I've always found Alaska Airlines the best amount of space for carriers. The fee is also cheaper than most airlines.