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Voltage Divider Rule Calculator

Voltage across a specific resistor in series.

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Result
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Output Voltage

Formula first

Overview

The Voltage Divider Rule is a fundamental circuit principle used to calculate the fractional voltage across one of two resistors connected in series. It describes how the total input voltage is distributed proportionally based on the resistance values in the loop.

Symbols

Variables

= Input Voltage, = Resistor 1, = Resistor 2, = Output Voltage

Input Voltage
Resistor 1
Resistor 2
Output Voltage

Apply it well

When To Use

When to use: Use this rule when dealing with series circuits to find the potential difference across a specific component without needing to calculate the total current first. It is most accurate when the output 'load' has a much higher resistance than the divider resistors, ensuring minimal current draw from the tap point.

Why it matters: This rule is the backbone of analog electronics, used to create reference voltages, bias transistors, and interface sensors like thermistors or LDRs with microcontrollers. It allows high-voltage signals to be scaled down to safe levels for processing by digital logic.

Avoid these traps

Common Mistakes

  • Using the wrong resistor in the numerator.
  • Convert units and scales before substituting, especially when the inputs mix V, Ω.
  • Interpret the answer with its unit and context; a percentage, rate, ratio, and physical quantity do not mean the same thing.

One free problem

Practice Problem

A 12V power supply is connected to a series circuit consisting of R1 = 200 Ω and R2 = 400 Ω. Calculate the output voltage across R2.

Input Voltage12 V
Resistor 1200 Ω
Resistor 2400 Ω

Solve for: vo

Hint: The output voltage is the input voltage multiplied by the ratio of R2 to the total resistance.

The full worked solution stays in the interactive walkthrough.

References

Sources

  1. Halliday, Resnick, and Walker, Fundamentals of Physics
  2. Wikipedia: Voltage divider
  3. NIST Special Publication 811: Guide for the Use of the International System of Units (SI)
  4. IUPAC Gold Book: electrical resistance, electric potential
  5. Halliday, Resnick, and Walker, Fundamentals of Physics, 11th ed.
  6. Halliday, Resnick, Walker Fundamentals of Physics, 10th Edition
  7. Wikipedia: Voltage divider (article title)
  8. AQA GCSE Physics — Electricity (P2)