download rpm counter 2006
count the number of pulses (p) coming from your sensor for a period 1 second. Depending the number of pulses per revolution (no of teeth of your disk): n
RPM=p/n*60
The 1 second delay can accurately adjusted with the help of a timer, and you may pull the sensor input. Or you can count the pulses using an external interrupt while looping in an accurately calculated 1 seond software delay loop.
You may shorten the 1 second delay for a quicker response. But for the sake of just display, I think the 1 second delay is satisfactory.