How to Calculate the Amps and Resistance of a Parallel Circuit

••• circuit board image by Gale Distler from

According to the Princeton University WordNet, a circuit is an electrical device that provides an avenue through which current can move. Electrical current is measured in amperes, or amps. The number of amps of current flowing through the circuit may change if the current crosses a resistor, which impedes current flow. In a series circuit, the current lessens with each resistor it crosses. In a parallel circuit, the resistors are placed in such a way that they are all receiving the same amount of current. Current and resistance can be calculated using Ohm's law.

Rtotal Method

    Calculate the total resistance of the parallel circuit by using the equation 1/Rtotal = 1/R1 + 1/R2 + 1/R3 + ... + 1/Rn. This equation states that by adding the inverses of all of the individual resistors, you will get the inverse of the total resistance. Pretend that you have two resistors in parallel, and each is four ohms. Rtotal equals 2 ohms.

    Identify the voltage of the system. Add the voltages together if two power sources in series are being used.

    Divide the voltage by Rtotal to determine the final value of current after it passes through the parallel resistors. This is Ohm's law, which can be written as I = V/Rtotal.

Additive Currents Method

    Identify the voltage of the system based on the power source being used. This will be provided or can be located on the power source itself, such as a battery label. Add voltages together if more than one power source is being used.

    Divide the voltage by R1 to get I1. V/R1 = I1. I1 will be measured in amps.

    Divide the voltage by R2 to get I2. Repeat this step for all resistors.

    Add all currents together that were calculated in Steps 2 and 3. There should be the same number of currents as there are resistors. This total is Itotal, and it is the final current coming out of the parallel circuit.


About the Author

Bailey Richert is a 2010 graduate of Rensselaer Polytechnic Institute with a dual bachelor's degree in environmental engineering and hydrogeology, as well as a master's degree in systems engineering. After several years in the environmental consulting industry, she is now attending MIT for graduate school. An accomplished traveler, she has visited 23 countries and published her first book about international travel in 2014.

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