It's pretty easy:

Cadence (in RPM) = 8.5 * MPH / W / G

Here, MPH is your speed in MPH, W is your wheel diameter (IN METERS, so 0.700 approximately), and G is your gear ratio.

To get your gear ratio, easiest thing is just to count teeth, so if the front chain wheel is a 53t, and the rear is 12t, you have a ratio of about 4.4.

Assuming a 700mm wheel, that would give:

CAD=MPH * 12.1 / G