To find circumference, you must know the radius or diameter of the circle. If you know the radius, circumference equals 2 times pi(3.14) times the radius. If you know the diameter
http://answers.ask.com/Science/Mathematics/how_to_...

1. Place the tape measure around your head, measuring slightly above the ears. 2. If you are measuring for a hat, consult a hat sizing chart. 3. If you are measuring your child&rsquo
http://www.ehow.com/how_2303125_measureheadcircu...

I dont think there is one. It is easier to do (Pi) x (radius) 2. Which can be simplified (to 8dp) to; 3.14159265 x (Radius) 2.
http://wiki.answers.com/Q/What_instrument_measures...
