Resistors protect your electrical circuits. This ensures that the electrical circuits do not overheat and burn by getting a larger voltage than required. Resistors are most commonly used in electronics, as they normally operate on 12 volts. The resistors are rated in ohms, so you must calculate the ohms required to figure out the proper size of the resistor that you need.

Determine the voltage flowing into the circuit. You can find this either on the power supply of a device or the battery. Most circuits use 12 volts.

Find the electric current required for your electrical circuit. You can find this on the circuit or in the manual of the devise you are going to power. The number is written in amperes.

Divide the voltage by the electric current to find the resistance, which is the ohms the resistor needs to be. For example, if you have a 12-volt circuit requiring 0.015 amperes of current, you will need a resistor with 333 ohms (12V / 0.015A = 0.333).