How to Calculate a DC Voltage Drop

Written by joe friedman
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How to Calculate a DC Voltage Drop
The voltage drop across this resistor is the product of its resistance and the current passing through it. (resistor image by Witold Krasowski from

When you work with direct current (DC) electricity, calculating a voltage drop follows Ohm's Law. The law states that voltage equals the product of current and resistance. Thus, to calculate DC voltage drop, you have two know both of those quantities. If you are calculating the voltage drop across a single resistor, you need only know that one resistor's resistance value. If you are instead calculating the voltage drop across a group of resistors, you need to know each of their individual resistances and calculate their equivalent combined resistance.

Skill level:

Things you need

  • Current in amps
  • Resistances of all resistors in Ohms

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  1. 1

    Calculate the equivalent combined resistance of all resistors concerned using Thevenin's Theorem (see Resources for help). The result is a single imaginary resistance whose value equals the cumulative effect of the actual group of resistors.

    If you only have one resistor, you can skip this step.

  2. 2

    Write down the current passing through the combined resistance. The current is the same entering one side of the group of resistors as it is when it exits out the other side.

  3. 3

    Multiply the combined resistance value from Step 1 by the current value from Step 2.

  4. 4

    The result is the DC voltage drop across the resistors involved.

Tips and warnings

  • If you don't know the resistance of an actual resistor, you can read it according to the code of the colour bands around it (see Resources).
  • Capacitors block DC current so they have infinite resistance. Inductors have zero resistance.

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