Pi is the ratio of a circle’s circumference to it’s diameter. This ratio has been calculated to be approximately 3.141592653. An engineer designing a large circular building would not need a measurement any more accurate than that, but mathematicians have calculated Pi to billions of digits.
∏ (The symbol for Pi, may not display correctly in some browsers)
Website with many digits of pi in a beautiful layout. http://3.14.maxg.org/