Voltage Divider Calculator
Compute output voltage, current, and power for resistor voltage divider circuits.
Compute output voltage, current, and power for resistor voltage divider circuits.
Results
A voltage divider produces an output voltage that is a fraction of its input voltage. Vout = Vin × R2 / (R1 + R2). Adding a load resistor in parallel with R2 reduces the output voltage.
Voltage Divider Calculator — Resistor Divider Online for Free
The voltage divider is one of the most used circuits in electronics. It steps down a voltage using two resistors in series, and it appears in sensor circuits, bias networks, level shifters, and power supplies. Our free Voltage Divider Calculator computes the output voltage, current, power dissipation, and voltage ratio instantly — with optional load resistance for real-world accuracy.
What Is a Voltage Divider?
A voltage divider is a simple circuit made of two resistors connected in series across a voltage source. The output voltage is taken from the junction between the two resistors. It is a direct application of Ohm's Law and Kirchhoff's Voltage Law.
The basic formula is:
Vout = Vin × R2 / (R1 + R2)
Where:
- Vin = input voltage applied across the entire divider
- R1 = the top resistor (between Vin and Vout)
- R2 = the bottom resistor (between Vout and ground)
- Vout = the output voltage at the junction
The output voltage is always a fraction of the input. If both resistors are equal, Vout is exactly half of Vin. If R2 is much larger than R1, Vout approaches Vin. If R1 is much larger than R2, Vout approaches zero.
| R1 | R2 | Ratio (Vout/Vin) |
|---|---|---|
| 1kΩ | 1kΩ | 50% |
| 1kΩ | 2kΩ | 66.7% |
| 2kΩ | 1kΩ | 33.3% |
| 1kΩ | 9kΩ | 90% |
| 9kΩ | 1kΩ | 10% |
How to Use the Voltage Divider Calculator
- Enter the input voltage (Vin) — the voltage applied across the full divider (e.g., 12V, 5V, 3.3V).
- Enter R1 — the top resistor value in ohms. Use whole numbers (e.g., 10000 for 10kΩ).
- Enter R2 — the bottom resistor value in ohms.
- Optionally enter a load resistance — if your divider feeds a circuit (not just a meter), the load draws current and changes the output voltage. Enter the load resistance to see the real-world output.
- Read the results — output voltage, current through the divider, power dissipated by each resistor, total power, and voltage ratio.
All results update live as you type.
The Effect of Load Resistance
The basic voltage divider formula assumes no current flows out of the Vout node — that nothing is connected to the output. In practice, any load connected to Vout draws current and pulls the voltage down.
When a load resistor (RL) is connected in parallel with R2, the effective R2 becomes:
R2effective = (R2 × RL) / (R2 + RL)
This is always less than R2 alone, which means the output voltage drops. The heavier the load (lower RL), the more the output drops.
| Load | Effect on Vout |
|---|---|
| No load (open circuit) | Vout matches the ideal formula |
| RL = 10 × R2 | Output drops by ~9% |
| RL = R2 | Output drops by ~33% |
| RL = R2 / 10 | Output drops by ~83% |
Rule of thumb: For the divider to maintain accurate output voltage, the load resistance should be at least 10 times R2.
Key Features
| Feature | Description |
|---|---|
| Live Calculation | Results update instantly as you change any input |
| Load Resistance | Optional field for real-world loaded divider calculations |
| Power Dissipation | Shows power for each resistor and total — critical for component selection |
| Current Display | Shows total current through the divider |
| Voltage Ratio | Displays the Vout/Vin percentage |
| Single Card UI | All inputs and results in one clean layout |
| Copy Values | Copy output voltage to clipboard |
Common Use Cases
Reading Analog Sensors
Many sensors (thermistors, photoresistors, potentiometers) produce a resistance that varies with the measured quantity. Placing the sensor as R2 in a voltage divider converts resistance to voltage, which a microcontroller's ADC can read.
Level Shifting Between Voltages
When connecting a 3.3V device to a 5V signal, a voltage divider can reduce the signal voltage to a safe level. For example, R1 = 2kΩ and R2 = 3.3kΩ with 5V input gives approximately 3.1V output.
Biasing Transistor Circuits
Transistors need specific base voltages to operate correctly. A voltage divider from the supply rail sets the DC bias point for the transistor's operating region.
Setting Reference Voltages
Many circuits need a reference voltage lower than the supply. A voltage divider provides a simple, inexpensive way to create one — though for precision applications, a dedicated voltage reference IC is preferred.
Volume Controls and Potentiometers
A potentiometer is essentially an adjustable voltage divider. The knob moves the wiper between R1 and R2, changing the output voltage from 0 to Vin.
Tips and Best Practices
- Keep divider current much higher than load current. For the output to remain stable, the current flowing through the divider (Vin / (R1 + R2)) should be at least 10 times the current drawn by the load.
- Check power ratings. The calculator shows power dissipation for each resistor. Choose resistors rated for at least double the calculated power to avoid overheating.
- Use standard resistor values. After calculating the ideal values, round to the nearest standard E24 values and verify the output voltage is still acceptable.
- Do not use voltage dividers as power supplies. Voltage dividers are meant for signal-level applications. They cannot supply significant current without the output voltage dropping. For power, use a voltage regulator instead.
- Account for tolerance. Resistors have manufacturing tolerance (typically 1% or 5%). Two 5% resistors can produce an output that varies by up to 10% from the calculated value.
Frequently Asked Questions
Is this Voltage Divider Calculator free?
Yes. The tool is completely free with no usage limits, no registration, and no hidden costs.
What units should I use for resistors?
Enter resistor values in ohms. For a 10kΩ resistor, enter 10000. For a 4.7kΩ resistor, enter 4700.
What is the load resistance field for?
When you connect something to the divider's output (like a microcontroller input or another circuit), it draws current and affects the output voltage. Enter the load resistance to see the actual output voltage under load. Leave it empty for the ideal unloaded calculation.
Can I use this for AC circuits?
This calculator is designed for DC circuits. For AC circuits, the impedance of capacitors and inductors must also be considered, which changes the divider ratio with frequency.
Why is my measured output voltage lower than calculated?
The most common reason is load current. If the circuit connected to the divider's output draws significant current, the output voltage drops below the ideal value. Enter the load resistance in the calculator to see the loaded output.
What is the voltage ratio?
The voltage ratio is Vout / Vin expressed as a percentage. A 50% ratio means the output is exactly half the input. This is useful for comparing different resistor combinations quickly.