I have a related question, which probably has a very simple answer but I really cannot find it.
How do you measure the circumference of a knitted hat, especially while it is being knitted? One author I read suggested the use of a flexible tape measure, but it is not easy to hold a tape exactly around the circumference of the hat, and at the same time spread out the hat, even with a second pair of hands. When knitting from the top down, it would be very handy to be able to tell when to stop making the increases.
I had the idea that the easiest would be to measure the diameter of the hat, then multiply by the value of pi (3.14), which is supposed to give the circumference of a circle. However, I have not found this advice in any knitting handbooks, so is there a catch? Is it because the head is not a perfect circle or what?