Convert the units of the known quantities on the circuit diagram to standard units, if necessary. Standard units are volts for voltage, ohms for resistance and amperes for current. Metric system prefixes are used for all these units, so consult a chart of metric prefixes if you need to convert. For example, divide by 1,000 to convert from millivolts to volts.
Multiply resistance by current to find voltage. If a circuit has a five-ohm resistor with two amperes of current flowing through it, for example, the power source must have 5 times 2, or 10 volts of power. Abbreviate volts with the uppercase letter V.
Divide voltage by resistance to find current. If a circuit has a 20-volt power source and a five-ohm resistor, for example, the current is 20 divided by 5, or four amperes. Abbreviate amperes with an uppercase A.
Divide voltage by current to find resistance. If a circuit has a 20-volt power source and 10 amperes of current flows through it, the resistor has 20 divided by 10, or two ohms of resistance. Abbreviate ohms with an uppercase Greek letter omega.