Is the blade length on a rapier determined from tip to the edge of the grip, or from the tip to the edge of the guard? The examples I have seen can make up to a 2 inch difference.

I think everybody measures it differently. I've done it different ways in the past. I've come to the point where I like to measure from the guard to tip and include a separate measurement for the ricasso, if present. Either that or I'll explicitly say what I'm measuring.

I generally do the same thing Nathan does. i.e. "A 40 inch blade with a two inch ricasso." But Nathan's right, people measure them in different ways. I know that in some period sources they measured all the way to the quillon (including the ricasso). The Spanish fencing master Narvaez said the perfect length for a rapier can be measured by setting the tip on the ground, and the quillons should reach the naval, so he clearly measured it to the quillon.

But if you ever decide to order a custom sword, definately be clear. I once ordered a custom rapier from A&A and requested a 43" blade... Craig later called me to double check whether I meant from the quillon or from where the blade comes out of the guard. It was a good thing he checked, because I meant from the guard, and we was going to do a 43" blade IN ADDITION to the ricasso, which would have been gigantic for me. :)

Length matters only insofar as it increases range, so I count everything below the quillon block. That's the maximum length you'd have to work with in a fight. In practice, you'd often reduce that length by an inch or two by gripping the ricasso with thumb and forefinger for better control.