NASA only uses 15 digits of pi for calculating interplanetary travel. At 40 digits, you could calculate the circumference of a circle the size of the visible universe to an accuracy equal to the diameter of a hydrogen atom.
In 2006, it took Akira Haraguchi, a retired Japanese engineer, 16 hours to recite the first 100,000 digits of pi from memory only. To prove that the exercise was legitimate, everything was filmed, even his trips to the toilet. Despite this, he has yet to receive a Guinness World Record.