There are a couple common methods of calculating RPMs, and which is best to use depends on a variety of factors.

1. Measure the time between pulses. Use that time to calculate RPMs.

If the time between pulses is measured in milliseconds, then RPM can be calculated using:

60000/time_between_pulses

2. Count the number of pulses that occur in an arbitrary period of time. Use that value to calculate RPMs.

If you count the number of pulses that occur in 1 second, then RPM can be calculated using:

pulse_count*60

Both these calculations assume you have 1 pulse per revolution (as you mentioned). Below are versions of the formula that add a pulses_per_rev parameter (which, when equal to one, will simplify to the above formulas)

60000/(time_between_pulses*pulses_per_rev)

(pulse_count/pulses_per_rev)*60

If, in the second formula you are counting the number of pulses in a period of time that is not 1 second, then you need to normalize the formula to a 1 second count period.