Motors run at a specific revolutions per minute (RPM) and a speed reducer is needed if you want to keep that motor, but have a lower RPM. Calculating the RPM resulting from a motor and speed reducer assembly requires only basic mathematical knowledge.
Write down the rated RPM of your motor; for example, 5000 RPM.
Write down the ratio of the speed reducer; for example, 12:5.
Calculate the reduction by dividing 12 by 5, which equals 2.4.
The RPM of the assembly is the RPM motor divided by the reduction. In our example, it would be 5000 RPM/2.4 = 2083 RPM.
The ratio of a reducer will always be in the form of A:B, where A>B.
The RPM found should always be lower than the base RPM of the motor. If it is not, either redo your calculation or make sure you did not get a speed multiplier instead of a reducer.