A voltage divider is one of the simplest and most used circuits in electronics: two resistors in series across a supply, with the output taken from the junction between them. The output voltage is a fraction of the input, set by the ratio of the two resistances, which makes dividers ideal for scaling a sensor signal down to a range an analog input can measure, or for creating a reference voltage. This calculator works in both directions. Give it the input voltage and both resistors to find the output, or give it the input, one resistor and a target output to solve for the second resistor. It also reports the divider current, which matters because a divider that draws too much current wastes power, while one that draws too little is disturbed by the load connected to it.
Quick Answer: Enter the input voltage and both resistors and click Output Voltage, or enter a target output and click Solve for R2 instead.
Free Online Tool
Voltage Divider Calculator
Output voltage from R1 and R2, or solve for R2 from a target.
How to Use the Voltage Divider Calculator
- Enter the input voltage that appears across the whole divider.
- For an output calculation, enter both resistor values in ohms.
- Click Output Voltage to see the voltage at the junction and the divider current.
- For a design calculation, enter one resistor and the target output voltage instead.
- Click Solve for R2 to find the second resistor value that produces that output.
Key Features
- Output voltage calculation from input voltage and two resistor values.
- Reverse solving for the second resistor from a target output voltage.
- Divider current reporting, showing how much current the divider itself draws.
- Ohm based inputs with standard unit handling for kilohm scale resistors entered as ohms.
- Instant local calculation for sensor scaling and reference voltage design.
Best Practices and Tips
- Keep divider current at least ten times the load current drawn from the output, so the load does not shift the output voltage noticeably.
- Use standard resistor values near the calculated result, then re check the output with the actual values you will install.
- Prefer higher resistor values for battery powered circuits, where divider current is wasted energy, but not so high that noise becomes a problem.
- Buffer the output with an op amp when a load must draw real current, since a bare divider is a poor power source.
Common Mistakes to Avoid
- Forgetting that connecting a load changes the divider, because the load appears in parallel with the lower resistor.
- Using a divider to power a circuit that draws significant current, which collapses the output voltage under load.
- Choosing resistor values without checking the divider current, leading to either wasted power or excessive noise sensitivity.
Frequently Asked Questions
How does a voltage divider work?
Two resistors in series share the input voltage in proportion to their resistances. The current through both is the same, so the voltage across each resistor is proportional to its resistance. The output, taken across the lower resistor, is the input voltage multiplied by the lower resistance divided by the total resistance.
Why does the load affect the output?
A load connected to the output sits in parallel with the lower resistor, reducing the effective lower resistance and therefore lowering the output voltage. The effect is small when the load draws far less current than the divider itself, and large when it draws a comparable current.
Can a divider power a device?
Only a device that draws almost no current, such as a high impedance analog input. A divider is a voltage reference, not a power supply: any significant load current shifts its output, so powered circuits need a regulator or a buffered reference instead.
What resistor values should I use?
Choose values that keep divider current reasonable for your application, typically in the kilohm to tens of kilohm range for signal work. Lower values waste power but resist noise and loading, while higher values save power but are more sensitive to both.