How to Calculate the Circumference of a Circle From the Diameter
- 1). Measure the diameter of the circle with a ruler, tape measure or other measuring tool. You must measure through the center of the circle to get an accurate value.
- 2). Enter the diameter into a calculator. The formula for the circumference of a circle is Circumference = pi * diameter, so multiply the measured diameter by pi. Use 3.1416 for the value of pi.
- 3). Read the answer, which is the circle's circumference. Example: for a diameter of 13.7 inches, the circumference is 13.7 * pi = 43.04 inches.
Source...