how to calculate RPM of brush less dc motor

Do you have a BLDC motor driver? Or there is a chance that the driver is built in to the motor circuitry internally.

There has to be a rotating magnetic field to make the rotor rotate in a brushless DC motor. Well, that is just the case with an Induction motor and synchronous motor too. The principle of working of BLDC motors are similar to synchronous machines. Let us place those aside for the time being.

The rotating field will pull the rotor along with it at the same speed and that is how electrical energy is converted to mechanical energy here. A rotating field is built up in brushless DC motors usually using Pulse Width Modulation. The stator poles are alternately energised at regular intervals to achieve this. Now, that is frequency - the 'f' part in the equation. You know everything else in the equation.

If the motor has a built in control circuit, you'll have to check its datasheet to get some insight about the controller and the maximum or rated rpm of motor. If it doesn't have a built in controller, make sure you check the datasheet and ensure the external controller is programmed to not drive it beyond the maximum rpm.

Speed of bldc depends on your driver circuit.at which frequency you feeding pulses to your bldc is decide the speed of rotor.