I am making a chemo cap. It's the first crocheting I've done in a very long time. I followed the instructions, but my gauge is not the same as the pattern maker assumes. So, the hat wasn't right (too small). Since I'm not an expert crocheter I decided I needed some help. I knew I wanted the hat to have about a 21.5" head circumference, but didn't know what the diamter should be. If I knew the diamter, then I could just keep making the top of the hat until it was the right size and then begin working downward toward the brim. I decided to see if I could find out what the diamter should be.
Geometry was never my strong point in high school. Back then, way back then, I couldn't figure out why I would ever need to know the diameter of a circle. Finally, there is a use for it! I found a website. I plugged the formula into an Excel spreadsheet and presto(!)... now I know the diamter should be almost 7", about 6 5/6". Who knew Pi could be your friend?