Look on your motor's nameplate to find the full load current in amps; i.e., the amount of current the motor draws when running at top speed. One example is a 16-amp full load.
Determine the voltage going to your motor. If you have it plugged into a standard household outlet, your motor will receive 120 AC voltage.
Multiply the current by the applied voltage to get the wattage. A 16-amp full load current will use 1,920 watts of power on a 120-volt line.
Divide the result by 746. A motor that uses 1,920 watts at a 16-amp full load current has an output of 2.6 horsepower.