How to calculate resistors in a circuit

Written by ronald bell
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How to calculate resistors in a circuit
Printed circuit board with many resistors. (printer circuit board image by Stephen VanHorn from

A resistor is an electronic component commonly found in electronic circuits. The "ohm" is the unit of measure for resistors. The value is usually indicated by colour-coded stripes on the resistor. Current flowing through a resistor causes a voltage drop across it. Resistors can be connected together in series---end to end, in parallel---side by side, or a combination of the two. When series and parallel configurations exist, the resulting effective resistance must be calculated. Reference 2 and Resource 1 below include diagrams that make it easier to visualise parallel and series resistor connections.

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  1. 1

    Identify those resistors whose effective resistance value is to be calculated. If the resistors are all connected in series, add their individual values. The total value is the total effective resistance. For example, three resistors in series with values of 120 ohms, 200 ohms and 600 ohms equal an effective resistance of 920 ohms. The formula for calculating the effective resistance "Re" for "n" resistors connected in series is Re = R1 + R2 + R3 + Rn. With multiple resistors in series, the effective resistance will always be larger than the largest individual resistor.

  2. 2

    Calculate the effective resistance of parallel resistors using the formula 1/Re = 1/R1 + 1/R2 + 1/R3 + 1/Rn. For example, three resistors in series with values of eight ohms, eight ohms and four ohms equal an effective resistance of two ohms. The equation is 1/R = 1/8 + 1/8 + 1/4, or 1/R = 4/8. In calculating the effective resistance of parallel resistors, the final answer will always be less than the smallest of the individual resistors.

  3. 3

    Locate and identify resistors that include both series and parallel combinations. To calculate these combinations, break them down by simplifying the circuit. Calculate for the parallel combinations, then calculate the parallel combinations. Continue this process until there is a single effective resistance. See Resource 1 to help visualise the process.

Tips and warnings

  • It often helps to redraw the circuit while reducing the series and parallel combinations. This allows you to reorganise the circuit on paper, so that combinations are easier to recognise.
  • Calculating resistors in a circuit is very difficult if you are working with a dense printed circuit board. Obtain a schematic of the circuit if possible before you begin.

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