In a little over a week I am flying LAX/SFO/STL and returning STL/IAH/LAX.
In F, with a carryon pet. The pet has logged a lot of miles, but all on mainline flights, and all in E+. When a GA starts in with "oh, I don't know if that will fit" and I'm boarding a 320 or anything Boeing, I have absolute confidence our carrier will fit, I can slide my messenger bag alongside the pet carrier under the seat, and relax and have a nice flight. I've never flown these two RJ's in any cabin by myself or with the animal, so I don't have that same brass confidence.

What is the underseat space like? SFO/STL is on the 175, STL/IAH is on a 170. In both cases I'm on the 2-seat side in F, up against the window. The pet carrier does meet UA's rules for size, and the top can flex down a few inches without interfering with the animal's comfort, but I need to know if it's going to fit or not!