text: The ratio of a circle 's circumference to its diameter is Ï ( pi ) , an irrational constant approximately equal to 3.141592654 . Thus the length of the circumference C is related to the radius r and diameter d by :
simplified: Ï can be measured by drawing a large circle , then measuring its diameter ( d ) and circumference ( c ) , since the circumference of a circle is always Ï times its diameter .