Calculating the circumference of your wrist
If you have measuring tape, you can measure you wrist much more easily than messing with the explanation below, so you can stop reading now!
If the only tool you have is your wrist, we can still help to approximate the circumference of your wrist by using this nifty formula 
and on-screen charts below (make sure the zoom in your browser is set at 100% - hit Ctrl-0 to reset it):
For the math geeks out there, the real oval circumference is calculated using Integrals, but we don't want to mess with them here, do we?
Imagine for a moment that you wear a Hemp Bracelet (ours of course, but works for others too) - it forms an oval or ellipse. All you need is to measure two dimensions of that oval (let's call it width and height) and plug them into the fields below.
It works equally well for anklets too! In fact, it kind of works for a neck too, but it might be a little hard to put your neck to the screen while paying attention to the size. You can ask you friend to measure it for you using a hard ruler.
Enter Width in inches:
Enter Height in inches:
Circumference:
Since this is "pure wrist size", you probably need to add about half an inch so it won't be too tight.