Ohm’s Law is one of the most useful laws you will find in electronics. It reveals that the voltage (potential difference) between two points is equal to the current multiplied by the resistance. A slightly more technical definition can be found on the Wikipedia Ohm’s Law page. It is usually expressed as I = V / R but I prefer to remember V = I x R and rearrange as required.

The calculator below allows you to specify two values and calculate the third.

Current is specified as milliamps as this is more convenient for small scale electronics. I use this calculator when deciding what resistors to use with my LEDs but it has lots of other applications as well.