To measure the circumference of a circle, first measure the diameter and multiply that number by the mathematical constant pi. The diameter is a straight line that goes from one side of the circle to the other and passes through the center, and the value of pi is always 3.141592.

Circumference is the distance around a circle. You can also determine the circumference of a circle by measuring the radius. The radius is a straight line that originates at the center of the circle and goes to any point on the edge of the circle. The radius is equal to half the diameter. If using the radius, the formula for calculating the circumference is C = 2rpi, or Circumference = 2 times the radius times pi.

The mathematical constant pi is the ratio of the circumference of a circle to its diameter. It is an infinite decimal, meaning that the numbers past the decimal go on forever and there is no exact value. However, the first three digits of pi are 3.14. For most equations this value is good enough because anything after the hundredths decimal place is tiny and therefore does not affect the magnitude of the final answer in most real world applications. Archimedes, an ancient Greek mathematician who lived between 287 and 212 B.C., was the first person to calculate pi.