If you really wanted to calculate this, CindySue... you would also have to take into account the gearing you were in... plus cadence... I think it would become quite problematic.

It is probably best to record it as minutes/hours ridden at 'x' resistence...

Otherwise, if they don't have some kind of speedo on the bikes in your class, can you just take your speedo along and attach the magnet to your spin bike for the session?