Right here: http://mp3.deepsound.net/eng/samples_calculs.php

This is a page of caculators that oughta help out any 505, 606, 808 heads (plus anyone else who uses software). Breaks down stuff like:
- find the bpm of a loop when its length in samples is known
- duration of a song based on tempo, beat and number of measures
- finding a note with its frequency

Cheers!

Thank you, this is looking really tidy. I'm sure I'll put this to use in my work