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Physics Tools · Updated October 21, 2026 · Reviewed by FormatFlow Editorial

Voltage Divider Calculator

You can calculate the output voltage of a two-resistor divider, or solve for the input voltage, R₁ or R₂, and see the divider ratio, current and power in each resistor.

Runs in your browserFree · No sign-up · No watermark
V_in, R₁, R₂ → V_outRuns in your browser
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This tool runs in your tab. Nothing is uploaded and no copy is stored.

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At a glance

Works with
V_in, R₁, R₂ → V_out
Processing
In your browser
Price
Free, no account
Updated
October 21, 2026
All Physics Tools

About Voltage Divider Calculator

A voltage divider uses two resistors in series to take a fraction of the input voltage. The output across the lower resistor is V_in × R₂ ÷ (R₁ + R₂), so the ratio depends only on the two resistance values as long as nothing draws current from the midpoint.

This calculator solves in four directions: output voltage from the input and both resistors, or input voltage, R₁ or R₂ from the other three values. It also reports the divider ratio as a percentage, the current through the pair and the power dissipated in each resistor, and marks the unloaded-divider assumption clearly.

What this tool does

The details that matter when you use Voltage Divider Calculator for real work.

  • Four solve directions

    Calculate V_out, V_in, R₁ or R₂ from the other three values using the same divider relation.

  • Divider ratio

    The R₂ ÷ (R₁ + R₂) ratio is shown as a decimal and as an output percentage of the input.

  • Current and power

    The current through the series pair and the power dissipated in R₁ and R₂ are reported with the result.

  • Resistance units

    Enter R₁ and R₂ in ohms, kilohms or megohms; the calculator converts to ohms before solving.

  • Formula steps

    The equation is printed with the actual values substituted, so the output voltage can be checked by hand.

  • Load warning

    A visible note explains that a real load across the output changes the delivered voltage.

How to use Voltage Divider Calculator

No account, no installation — the whole job happens in this browser tab.
  1. 01

    Choose what to calculate

    Select output voltage, input voltage, R₁ or R₂ depending on the missing value in the divider.

  2. 02

    Enter the known voltages and resistors

    Voltages are in volts; both resistors use the selected ohm, kilohm or megohm unit.

  3. 03

    Mind the passive-divider limits

    When solving for a resistor, keep the output below the input so the result is physically possible.

  4. 04

    Read the output and ratio

    The headline gives the solved quantity, and the summary shows the output percentage, current and power.

  5. 05

    Check the working

    The formula panel substitutes V_in, R₁ and R₂ into the divider equation, then Reset returns an empty divider.

How this tool works

Works with
V_in, R₁, R₂ → V_out
Processing
Client-side, in your browser. Nothing is uploaded.
Requirements
Chrome, Edge, Firefox or Safari · JavaScript enabled

Limitations worth knowing

  • Assumes an unloaded divider; a real load across the output lowers the delivered voltage.
  • Treats the resistors as ideal and the supply as DC; reactive impedances and transients are outside the model.
  • Is not a regulator or a safety tool: loading, tolerance, power ratings and heat need separate checks.

Privacy and file handling

Voltage and resistance values are processed entirely in your browser. Nothing is uploaded, stored or logged, and no network request is made.

Read the full privacy policy

Voltage Divider Calculator FAQ

Short, direct answers to the questions people ask before using this tool.
What is a voltage divider and what is its formula?
A voltage divider is two resistors in series with the output taken between them, usually across the lower resistor. The formula is V_out = V_in × R₂ ÷ (R₁ + R₂), where R₁ is the upper resistor and R₂ is the lower one.
How do I choose R₁ and R₂ for a divider?
The ratio R₂ ÷ (R₁ + R₂) sets the output, while the absolute values set the standing current. Very large resistors reduce current but make the output easier to disturb, and very small resistors waste power, so the choice is a balance between loading and dissipation.
Why does a load change the output voltage?
A load across the output appears in parallel with R₂, which lowers the effective lower-leg resistance. That reduces the divider ratio and pulls the output voltage down, which is why the calculated value is the unloaded result and a real circuit needs the load included.
Why can the output not exceed the input in this calculator?
A passive divider only divides the input, so its ratio is at most one and the output cannot be larger than the supply. The calculator rejects voltage pairs where the output is equal to or greater than the input when solving for a resistor, because those values would need a negative resistance.
What current flows through a voltage divider?
With nothing connected to the midpoint, the same current flows through R₁ and R₂ and equals V_in ÷ (R₁ + R₂). Adding a load draws extra current from the midpoint, so the current through the upper resistor becomes larger than the current through the lower one.
How do I calculate power dissipation in the divider resistors?
Use P = I² × R for each resistor with the standing divider current. In a high-resistance divider the powers are small, but a low-resistance divider can dissipate noticeable heat, so check both resistor ratings with margin for the actual supply voltage.
Can a voltage divider be used with a sensor?
Yes, a resistive sensor such as a thermistor or photoresistor is often one leg of a divider. The output then varies with the sensor resistance, but the response is nonlinear and the measurement circuit can load the divider, so calibration and buffering are usually required.
How do I find the input voltage from a known output voltage?
Rearrange the formula to V_in = V_out × (R₁ + R₂) ÷ R₂. Selecting the input-voltage mode does this automatically: enter the desired output and both resistors, and the calculator returns the supply voltage that would produce that output in an unloaded divider.

Background reading

Format guides that explain the trade-offs behind Voltage Divider Calculator.