Voltage Divider Calculator
This voltage divider calculator computes the output voltage of a two-resistor divider from your input voltage and resistor values, using the rule Vout = Vin × R2 / (R1 + R2). It is built for electronics hobbyists, students, and engineers who need fast, accurate divider math.
Work in either direction: enter Vin, R1, and R2 to find Vout, or set a target output voltage and let the tool find the resistor you need — complete with the nearest E24 standard value. You can also turn on a load resistance to see how a connected circuit pulls the real output away from the ideal.
How to Use the Voltage Divider Calculator
Pick a mode
Choose Calculate Vout to find the output from known resistors, or Find Resistor to work backward from a target output voltage.
Enter your values
Type the Input Voltage and pick its unit (mV, V, or kV). Then enter R1 and R2 in Ω, kΩ, or MΩ. In Find Resistor mode, enter the Desired Output Voltage and the one resistor you already have.
Add a load (optional)
Toggle Load Resistance on and enter the load value to compare the loaded output with the ideal no-load output. A warning appears when the load shifts Vout by more than 5%.
Read the results
The output voltage, voltage ratio, total current, and power in each resistor update instantly, alongside a live circuit diagram. Adjust Precision from 2 to 6 decimals to match the detail you need.
Features
Calculate Output Voltage
Find Vout instantly from an input voltage and two series resistors using the standard voltage divider rule.
Find Required Resistor
Set a target output voltage and the resistor you already have, and the tool solves for the matching R1 or R2.
E24 Standard Suggestions
Get the nearest E24 series value, the actual Vout it produces, and the percentage error versus your target.
Load Resistance Analysis
See how a connected load lowers the output, with a warning whenever the deviation exceeds 5%.
Live Circuit Diagram
An SVG schematic redraws with your real values, showing Vin, R1, R2, Vout, current, and the load when enabled.
SI Prefix Units
Switch each field between mV/V/kV for voltage and Ω/kΩ/MΩ for resistance, with results auto-scaling for readability.
Full Circuit Analysis
Beyond Vout, view the voltage division ratio, total current, and power dissipated in R1 and R2.
Adjustable Precision
Choose 2 to 6 decimal places so results read cleanly for quick estimates or fine-tuned designs.
Quick Example Presets
Load common circuits like 5V to 2.5V or 9V to 3.3V logic with a single click to start from a working setup.
Formula Reference
Keep the voltage divider formulas on hand while you work, so the math behind each result is always clear.
Frequently Asked Questions
What is the voltage divider formula?
The basic voltage divider formula is Vout = Vin × R2 / (R1 + R2), where Vin is the input voltage, R1 is the top resistor connected to Vin, and R2 is the bottom resistor connected to ground. The output is taken at the junction between R1 and R2.
How do I calculate Vout from R1 and R2?
Stay in Calculate Vout mode, enter the input voltage, then enter R1 and R2 with their units. The calculator multiplies Vin by the ratio R2 / (R1 + R2) and shows the output voltage, division ratio, current, and power in each resistor as you type.
How do I pick R1 and R2 for a target voltage?
Switch to Find Resistor mode, enter the input voltage and your desired output, then choose which resistor you already have and enter its value. The tool solves for the other resistor and suggests the nearest E24 standard value with its actual output and error. Many pairs give the same ratio, so values from 1kΩ to 100kΩ suit most applications: lower values regulate the load better but use more power, while higher values save power but are more sensitive to loading.
Does load resistance affect a voltage divider?
Yes. The basic formula assumes no current is drawn from the output. Any connected circuit places a resistance in parallel with R2, which lowers the effective ratio and the output voltage. Enable Load Resistance to see the loaded Vout next to the ideal value; if the load shifts the output by more than 5%, the tool warns you and suggests using lower R1/R2 values.
How do I divide 5V down to 3.3V?
Set Vin to 5V and use Find Resistor mode with a desired output of 3.3V, or simply test values in Calculate Vout mode. A common solution is R1 = 1.7kΩ and R2 = 3.3kΩ (or an E24 approximation), which produces roughly 3.3V. Because real microcontroller inputs draw little current, a divider is fine for logic-level reading, but turn on Load Resistance to confirm the loaded output stays accurate.
What are E24 standard resistor values?
E24 is a series of 24 preferred values per decade (1.0, 1.1, 1.2, 1.3, 1.5, 1.6, 1.8, 2.0, 2.2, 2.4, 2.7, 3.0, 3.3, 3.6, 3.9, 4.3, 4.7, 5.1, 5.6, 6.2, 6.8, 7.5, 8.2, 9.1), repeated in multiples of ten. Most commercial resistors with 5% tolerance follow these values, so the calculator suggests the nearest E24 part you can actually buy.
Can a voltage divider increase voltage?
No. A resistor voltage divider can only reduce voltage, so the output is always less than or equal to the input. To step a voltage up, you need an active circuit such as a boost converter or charge pump.
Is my data private?
Yes. Every calculation is performed locally in your browser with JavaScript. No voltages, resistor values, or results are ever sent to a server.
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