To find the amperage of a current based on the number of millivolts, you will need to know the number of watts produced by the current. Once you know the number of millivolts and watts, you can use the basic power conversion formula "Watts = Volts x Amps" to find the number of amps. You will need to convert millivolts to volts before using the formula.
Determine the wattage rating of the device powered by the electrical circuit. The wattage is most often listed somewhere on the device itself, or in its user manual.
Divide the number of millivolts by 1,000 to find the number of volts in the circuit. Each volt contains 1,000 millivolts.
Divide the number of watts by the number of volts. The result will be the amperage, or number of amps, in the circuit.