You are seeing this majestic tiger because you have chosen to disable Javascript in your browser. This page uses Javascript to perform and display calculations, so you must let it run in order to use the page. If you do not want to disable Javascript, you can follow the Instructable to get the equations to carry out your calculations.
All measurements must use same units. Doesn't matter which units, as long as they're uniform. You can perform calculations using the diameter or the circumference, whichever is easier for you to measure.

I'm just a grad student, standing in front of a vinyl cutter, asking it to do my creative bidding. I'm just a maker, standing in front of an html file, enjoying the fact that automation makes our work easier.
Here's the Instructable that this page is intended for. Here's my website.