PhysicsCalcs
Electricity

Voltage Divider Calculator

Calculate output voltage, resistor values, divider current, power dissipation, and loaded output for a two-resistor voltage divider circuit. Learn how resistor ratios control voltage.

Interactive calculator

Voltage Divider Calculator

Calculate output voltage, resistor values, divider current, power, and loaded output for a two-resistor voltage divider.

Supply voltage

Resistor from Vin to output node

Resistor from output node to ground

Your result will appear here.

Choose a calculation mode, fill in the known values, and click Calculate.

Quick Guide

  • Choose what to calculate from the dropdown.
  • Enter the known values and units.
  • Click Calculate to see the result.
  • Use the Loaded Divider mode to see how a load affects Vout.

Table of Contents

Key Takeaways

  • A voltage divider uses two resistors to create a lower output voltage from a higher input voltage.
  • Output voltage depends on the ratio R2/(R1+R2).
  • Loaded voltage dividers produce lower output than the unloaded formula predicts.
  • Higher resistor values reduce current but make the divider more sensitive to load.
  • Resistor tolerance affects actual output voltage.
  • Voltage dividers are useful for signals, sensors, and reference voltages, but not ideal for powering heavy loads.

What Is a Voltage Divider?

A voltage divider is a simple circuit with two resistors in series that produces an output voltage that is a fraction of the input voltage. The output is taken from the junction between the two resistors.

Voltage dividers are used extensively in electronics for signal scaling, sensor interfaces, biasing, and reference voltages. They rely on the fact that voltage across series resistors divides proportionally to resistance.

Voltage Divider Infographic explaining Vout = Vin * (R2 / (R1 + R2))

Voltage Divider Formula

Vout=Vin×R2R1+R2V_{\text{out}}=V_{\text{in}}\times\frac{R_2}{R_1+R_2}

Output voltage equals input voltage multiplied by the ratio of R2 to the total resistance.

Where R1 is the top resistor (from Vin to the output node) and R2 is the bottom resistor (from the output node to ground). Vout is measured across R2.

Calculate Resistors for Target Vout

R1=R2×Vin−VoutVoutR_1=R_2\times\frac{V_{in}-V_{out}}{V_{out}}

Find R1 when R2 and target Vout are known.

R2=R1×VoutVin−VoutR_2=R_1\times\frac{V_{out}}{V_{in}-V_{out}}

Find R2 when R1 and target Vout are known.

Choose one resistor value and calculate the other. Then select the nearest standard resistor value for the calculated result.

Loaded Voltage Divider

When a load resistor RL is connected across R2, it acts in parallel with R2, reducing the effective bottom resistance. This lowers the output voltage compared to the unloaded case.

R2′=R2×RLR2+RL,Vout=Vin×R2′R1+R2′R_2' = \frac{R_2 \times R_L}{R_2 + R_L}, \quad V_{\text{out}} = V_{\text{in}} \times \frac{R_2'}{R_1 + R_2'}

The load changes the effective resistance and output voltage.

For stable output, load resistance should be much larger than R2 (typically at least 10×). Otherwise, use a buffer amplifier or voltage regulator.

How to Use the Voltage Divider Calculator

  1. 1Choose what to calculate: output voltage, R1, R2, or loaded divider.
  2. 2Enter the known values with units.
  3. 3Click Calculate to see the result.
  4. 4Review the formula breakdown, divider current, and power.

Example Calculations

Basic divider

Vin = 5 V, R1 = 10 kΩ, R2 = 10 kΩ. Vout = 5 × 10k/(10k+10k) = 2.5 V.

Target 3.3 V from 5 V

Vin = 5 V, Vout = 3.3 V, R2 = 10 kΩ. R1 = 10k × (5−3.3)/3.3 ≈ 5.15 kΩ.

Loaded divider

Vin = 5 V, R1 = 10 kΩ, R2 = 10 kΩ, RL = 10 kΩ. R2' = 5 kΩ. Vout = 5 × 5k/(10k+5k) = 1.67 V instead of 2.5 V.

Divider current

Vin = 12 V, R1 = 10 kΩ, R2 = 20 kΩ. I = 12/30k = 0.4 mA.

Divider Current and Power

Divider current flows through both resistors: I = Vin/(R1 + R2). Each resistor dissipates power: P = I²R. Check power ratings in real circuits.

Common Voltage Divider Mistakes

  • • Using a voltage divider to power heavy loads without a buffer or regulator.
  • • Ignoring the effect of load resistance on output voltage.
  • • Confusing R1 and R2 positions (R1 is on top, R2 on bottom).
  • • Using very low resistor values and wasting power.
  • • Using very high resistor values, making the divider noise-sensitive.
  • • Forgetting resistor tolerance when precision matters.

Where Voltage Dividers Are Used

Voltage dividers are used in sensor circuits, Arduino/microcontroller analog inputs, reference voltages, battery voltage measurement, signal scaling, simple biasing circuits, physics homework, and electronics learning.

Accuracy and Limitations

This calculator assumes ideal resistors. Real resistors have tolerance and temperature coefficients. Voltage dividers are not suitable as power supplies for significant loads. For stable power output, use a voltage regulator. This tool is for educational and basic circuit calculations.

Frequently Asked Questions

What does a voltage divider calculator do?›

It calculates the output voltage from two series resistors, or helps find resistor values for a target output voltage. It also analyzes loaded dividers.

What is the voltage divider formula?›

Vout = Vin × R2/(R1 + R2). R1 is the top resistor from Vin to the output node, and R2 is the bottom resistor from the output node to ground.

How do I calculate output voltage from two resistors?›

Multiply the input voltage by the ratio of the bottom resistor to the total resistance: Vout = Vin × R2/(R1+R2).

How do I choose R1 and R2 for a target voltage?›

Fix one resistor and calculate the other. For example, R1 = R2 × (Vin − Vout)/Vout.

Why does load resistance change Vout?›

A load in parallel with R2 reduces the effective bottom resistance, changing the divider ratio and lowering the output voltage.

Can I use a voltage divider as a power supply?›

Generally no. Voltage dividers can’t maintain a stable output under significant load. Use a voltage regulator for power delivery.

What is the difference between R1 and R2?›

R1 is the top resistor (from input to output node) and R2 is the bottom resistor (from output node to ground). Vout is measured across R2.

Do resistor tolerances affect output voltage?›

Yes. A 5% tolerance on each resistor can shift the output voltage by several percent. Use tighter-tolerance resistors for precision.

How much current should a voltage divider use?›

Enough to maintain stability under load, but not so much that it wastes power. The load current should typically be much less than the divider current.

Can I use this calculator for Arduino voltage sensing?›

Yes. Voltage dividers are commonly used to scale higher voltages down to the 0–5 V or 0–3.3 V range of Arduino analog inputs.

Sources / References

  1. OpenStax College Physics 2e: Resistors in Series and Parallel
  2. NIST: SI Units
  3. HyperPhysics: Voltage Divider
Manish Kumar

Author & technical reviewer

Manish Kumar

PhysicsCalcs tools are reviewed with an educational focus: clear formulas, transparent assumptions, and practical context for students and science learners.

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