where Vs is the power supply voltage, e.g. a 9-volt battery; Vf is the LED forward voltage (also referred to as the "voltage drop") across the LED; and I is the desired current of the LED. Both Vf and the maximum recommended value for I are obtained from the LED manufacturer's specifications. The above formula requires the current in amperes, although this is usually given by the manufacturer in milliamperes, such as 20 mA.