Spirit Personal Bag?

WILL they measure it?

Probably not.

CAN THEY? Yes

If you do not cram it full to bursting capacity it can be easily squeezed in to most spaces.

This is a low seat price Airline they make MONEY hauling stuff. Paying in advance is cheaper than at the gate if measured.

They will not bother to measure it.

If it's not stuffed all the way full, then you are probably going to be okay.  But I will say that my sister had a backpack that was slightly over the allowed dimensions (but DID fit easily under the seat in front of her), and Spirit measured it at the gate and forced her to pay the carry-on fee (which is more at the gate than it is if you pay it in advance). If you have a bag that is within their dimensions, it will be better to take it.

relax, they wont measure it

I'd cal them. You don't want to be stuck at the airport deciding between leaving your back pack, paying for a checked bag, or not getting on the plane!.

yes....that is great....

They might but my experience is they won't. I HAVE seen them weighing carry-on bags on International flights though we've never been checked (older couple).

they might,i would ask the airlines about it or get another bag