Traveling on the Embraer ERJ-170 and wondering if a toddler car seat will fit?
 

will it fit comfortably?

Carrier? Class of service? Which carseat? The Cosco Finale is very different from a Britax Advocate?

As said by FindAWay, it is important to check with he airline company: each plane as specific dimensions regarding the seats and which car seat fits. It should be available on their website. Otherwise a quick call.

On top of that, you have to make sure your carseat is approved for flight, either FAA, or other certification for the plane.

I've flown with a Cosco Scenera on E-145s, and they fit. If you have a bulky car seat, I recommend buying a smaller one (the Scenera is great for this) for plane travel, not only for making life easier for you when you install the seat, but also for hauling it through the airport. And as someone said above, make sure it's FAA certified.

Also, make sure to practice installing the seat with a lap-only belt. It can be a bit of a pain on airplanes, but once you get the hang of it, you're good.

Finally, if there's a lot of airport transitioning, I highly recommend a car seat trolley. You strap the seat to it, and push seat+toddler comfortably through the airport. They fold small enough to fit almost every overhead compartment.

[Just realized this is an older post -- hopefully above will be useful for others.]