Voltage Divider Calculator
Calculate voltage divider values
Calculate Vout, R1, or R2
Circuit diagram
Formula display
Your sensor outputs 0-5V but your microcontroller only accepts 0-3.3V. The reference voltage needs to be exactly 2.5V from a 5V supply. You need to measure a 12V battery with a 3.3V ADC. All these problems have the same solution: a voltage divider.
Voltage dividers are one of the most common circuits in electronics. Two resistors, predictable output. This calculator sizes those resistors correctly.
What is a Voltage Divider?
A voltage divider uses two series resistors to produce an output voltage that's a fraction of the input voltage. The ratio of resistors determines the output voltage.
The formulas:
Vout = Vin × R2 / (R1 + R2)
To find resistors for desired ratio:
R2 / (R1 + R2) = Vout / Vin
Example: 5V to 3.3V
R1 = 1.7kΩ, R2 = 3.3kΩ (or ratio like 1.7:3.3)
10kΩ and 20kΩ give the same ratio as 1kΩ and 2kΩ. But absolute values affect current draw and loading effects.
Why People Actually Need This Tool
Different components operate at different voltages. 5V sensors, 3.3V microcontrollers, 12V batteries, 1.8V processors—they all need to communicate.
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Microcontroller input protection — Scale higher voltages to safe ADC range.
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Reference voltage generation — Create precise voltages from power supply.
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Sensor interfacing — Match sensor output to controller input.
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Battery monitoring — Measure high-voltage batteries with low-voltage ADC.
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Logic level conversion — Interface 5V and 3.3V systems.
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Bias voltage generation — Create operating points for analog circuits.
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Volume/brightness controls — Potentiometers are adjustable voltage dividers.
How to Use the Voltage Divider Calculator
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Enter input voltage — The voltage you're dividing down.
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Enter desired output — The voltage you need.
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Or enter resistor values — Calculate output from known resistors.
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Get results — Resistor values or output voltage, plus current draw.
| Application | Vin | Vout | Typical R1 | Typical R2 |
|---|---|---|---|---|
| 5V → 3.3V | 5V | 3.3V | 1.7kΩ | 3.3kΩ |
| 12V battery monitor | 12V | 3V | 30kΩ | 10kΩ |
| 9V → 5V reference | 9V | 5V | 8kΩ | 10kΩ |
| 24V → 3.3V | 24V | 3.3V | 62kΩ | 10kΩ |
A divider's output voltage drops when you draw current from it. For significant current, use a voltage regulator instead.
Real-World Use Cases
1. The Sensor Interface
Context: Ultrasonic sensor outputs 0-5V. Raspberry Pi GPIO is 3.3V max.
Problem: Scale 5V signal to safe 3.3V range.
Solution: R1 = 1kΩ, R2 = 2kΩ. Vout = 5V × 2k/(1k+2k) = 3.33V.
Outcome: Pi safely reads sensor without damage.
2. The Battery Monitor
Context: Monitoring 12V car battery with 3.3V ESP32 ADC.
Problem: 12V will destroy the ADC input.
Solution: R1 = 27kΩ, R2 = 10kΩ. Vout = 12V × 10k/37k = 3.24V max.
Outcome: Full battery range fits ADC input range safely.
3. The Reference Voltage
Context: Need 2.5V reference for comparator from 5V supply.
Problem: Create stable 2.5V with resistors.
Solution: R1 = R2 = 10kΩ. Vout = 5V × 10k/20k = 2.5V exactly.
Outcome: Clean reference voltage for analog circuit.
4. The Potentiometer Application
Context: Volume control for audio amplifier.
Problem: Need variable voltage from 0V to signal level.
Solution: Potentiometer IS a variable voltage divider. Wiper outputs 0 to Vin.
Outcome: Understanding that pots are dividers in action.
5. The High Voltage Measurement
Context: Measuring 48V solar panel with 5V ADC.
Problem: Scale 48V to 0-5V safely.
Solution: R1 = 86kΩ, R2 = 10kΩ. Vout = 48V × 10k/96k = 5V max.
Outcome: Solar voltage monitored without ADC damage.
6. The Bias Point
Context: Amplifier needs 1.65V bias from 3.3V supply.
Problem: Create bias at exactly half of supply.
Solution: Two equal resistors. R1 = R2 = 47kΩ. Vout = 1.65V.
Outcome: Amplifier biased at mid-supply for maximum swing.
7. The Current Limiting Consideration
Context: Need 3V reference that can drive 1mA load.
Problem: Low-resistance divider wastes power, high-resistance can't drive load.
Solution: Calculate: At 1mA load, 1kΩ source resistance drops 1V. Use 100Ω resistors (but wastes 30mA standing current).
Outcome: Understanding trade-off between loading and power consumption.
Common Mistakes and How to Avoid Them
A voltage divider's output impedance equals R1||R2. Drawing current from it changes Vout.
Privacy and Data Handling
This Voltage Divider Calculator operates entirely in your browser.
- No calculations are sent to any server.
- No circuit designs are stored.
- No account required.
- Works completely offline.
Your electronics work stays private.
Conclusion
Voltage dividers are fundamental building blocks—simple, predictable, and useful. Two resistors can scale any voltage to any other voltage (lower), create reference points, and interface incompatible systems.
This calculator eliminates the math. Enter your voltages, get resistor values. Or enter resistors, see the output. Either way, design voltage dividers correctly the first time.
Simple circuit. Powerful tool.