How to Calculate Missing an Ohm on a Parallel Circuit

Written by jonah quant
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How to Calculate Missing an Ohm on a Parallel Circuit
Calculating the missing resistance in a parallel resistive circuit is a straightforward process. (resistor image by Witold Krasowski from

Many electrical circuits have multiple resistive components connected in parallel with one another. In a parallel connection, one terminal from each resistor is connected to another and to the rest of the circuit; all remaining terminals from each resistor are also connected together and to another point in the rest of the circuit. The set of resistors connected in parallel behave as a single resistor. Even when the resistance (measured in Ohms) of a resistor in a parallel ensemble is known, determining the resistance of the other resistor is a common problem in electrical engineering.

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Things you need

  • Multimeter

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  1. 1

    Set the multimeter in "resistance-measuring" mode.

  2. 2

    Disconnect the parallel ensemble from the rest of the circuit. Measure the end-to-end resistance of the parallel ensemble using the multimeter. Call that measurement (in Ohms) "R_total."

  3. 3

    Calculate the resistance of the unknown resistor, "R_unknown," by utilising this formula: R_unknown = (R_total x R_known) / (R_known - R_total). Replace "R_known" with the known resistance of the other resistor in the parallel ensemble and work the equation.

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