Pi is famously calculated to trillions of digits - but Dr James Grime says 39 is probably enough. All our Pi videos: http://bit.ly/W4oDN1
An extra note from Dr Grime: "Since pi39 ends in 0, you may think we could use pi38 instead - which has even fewer digits. Unfortunately, the rounding errors of pi38 are ten times larger than the rounding errors of pi39 - more than a hyrdogen atom. So that extra decimal place makes a difference, even if it's 0."