Mathmatical Forumlas in Families

Not a math genius, or even a Revit one (or else I would have spent way less time figuring this out) but I think I have a way to deal with the "Inconsistent Units" error message. I had a problem in which I needed to calculate the arc lengths of some segments. I used a slightly different method (from previous posters, Ed), but the critical piece (the "Inconsistent Units" error) was the same. In the end I realized that Revit, just like my Chemistry teacher, expects me to cancel units in my equations. Even though WE know that the units cancel out (and that Pi is unit-less) Revit is quite literal: Pi is a number, arc length is an angle, radius is a Length and so on. It cannot see how you could possibly relate them with mathematics.

As you realized, you can trick Revit in a couple of ways, I am attaching a screen shot of two examples. In the first (Green Arrow) I added incorrect units to values. You can see this makes the equation "work" but yields an answer in degrees whilst we want feet and inches. The second way, (red Arrow) is to break up your equations so that portions of them yield "unit less" values, and then combine them at will. Of course you could get this functionality in a single equation, so long as your units all cancel to yield whatever parameter type you are supposed to have.