The circumference of a circle with a 40 foot diameter is 125.66 feet.

The formula to calculate the circumference is C = 2pr, or pd, where C is the circumference, r is the radius, d is the diameter and p is the mathematical constant that is approximately 3.14. The area of a circle is equal to p times the radius squared. p is an irrational number, meaning the decimal is not repeating and it never ends.

Modern mathematicians have been able to extend the decimal out 10 trillion digits. Pi Day is unofficially celebrated on March 14th (3/14).