Can You Write a Formula for This?

We're constantly asked "how much rope do I need for this job?" I really can't calculate all of these jobs without risking my head exploding. This is because I'm not a "pro bono" consultant, and also because there are so many variables involved.

Of course, I know there's a cat lover out there who has the necessary mental resources to toss off a formula to solve this ( like it was just staring you in the face).

All we need is an elegant equation for someone trying to wrap a round column from say, 2 to 16 inches. If you can come up with one for 2x4's, 2x6's, etc... that would be a bonus!

Can you lend a hand & solve this nagging question for your fellow do-it-yourself-ers? If you are, I am really happy to post such an equation here to help out. As usual, there is no budget here for your professional services. But we've always had a liberal budget to provide our thanks.