Either pi times the diameter or pi times twice the radius (which is the diameter)
http://wiki.answers.com/Q/What+is+the+formula+for+...

How to find the circumference of a circle<br/><br/>First here are some geometry basics:<br/>Circumference is the distance all the way around a circle.<br/>
http://answers.ask.com/Science/Mathematics/how_to_...

Mathulon, our resident mathmagician says the formula to
http://www.chacha.com/question/whatistheformula...

If you already have a given diameter,The line that cuts the circle in half, then you have everything you need! Make sure the line goes all the way from one end of the circle to the
http://www.ehow.com/how_4748215_circumferencecirc...
