I know it is not such a big deal at all, and is implemented in most DAW software, but not for iphone/ipad. If you want to put some of your music creation from iphone music apps to your movie on iphone you got to know this.
Didint find any iphone app to do the calculation - an idea for new music utlily app for iphone maybe
Found song time app for windows which is great from analogx.com
The formula is not complcated:
"Number of measures x BPM = total Beats
Total Beats / BPN = Total Time
The answer is in minutes and a decimal. To change the decimal to seconds, multiply by .60."
by Jerry Engelbach on jazz forum