Pi is famously calculated to trillions of digits -- but how many of them do we really, really need? This video demonstrates that just 39 will do.
Taking pi to 39 digits allows you to measure the circumference of the observable universe to within the width of a single hydrogen atom. Sure, you could use more, but it's not really worth the effort, and that means that on a day-to-day basis you can ignore far more of them. [YouTube]