You don't need to get yourself a computer...

here are some quick ways to measure:

ok so real quick - count number of revolutions on one side in 6 seconds, add a 0, that's roughly your cadence

a little more accurate - count number of revolutions of one leg (one side) for about 15 seconds, multiply by 4.

You want to pedal at a comfortable rate, with some tension in the chain, and without bouncing in the saddle. most people pedal naturally around 80-90 on average, but some people are gear mashers, and some pedal quicker.

SPIIIIIIIIIIIN!

Han