DC Motor measurement

I need to know the VELOCITY or the RPM of the dc motors I have attached to my Arduino. Does someone know any conversion formula or way?

Thanck you

The speed depends on the load, the supply voltage and the temperature of the windings. Speed is measured, not calculated, with a DC motor. A rough estimate might be possible, but only if the motor is lightly loaded.

You'll need a rotary encoder... just search for it and many examples will appear.

1 RPM = 0.1 radian/sec

any questions?