Voltage Divider Calculator Formula
Understand the math behind the voltage divider calculator. Each variable explained with a worked example.
Formulas Used
Output Voltage
v_out = v_in * r2 / (r1 + r2)Division Ratio
ratio = r2 / (r1 + r2)Variables
| Variable | Description | Default |
|---|---|---|
v_in | Input Voltage(V) | 12 |
r1 | Resistor R1 (top)(Ω) | 10000 |
r2 | Resistor R2 (bottom)(Ω) | 10000 |
How It Works
How a Voltage Divider Works
Two series resistors produce an output voltage that is a fraction of the input.
Formula
V_out = V_in x R2 / (R1 + R2)
The output is accurate only when load impedance is much higher than R2.
Worked Example
Divide 12 V using two equal 10 kohm resistors.
- 01V_out = 12 x 10000 / (10000 + 10000)
- 02V_out = 12 x 0.5 = 6.0 V
When to Use This Formula
- Stepping down a sensor output voltage to a safe level for a microcontroller ADC input — for example, converting a 0-12V signal to 0-3.3V.
- Biasing a transistor base or gate to a specific operating voltage in an amplifier circuit.
- Creating a reference voltage from a higher supply when a dedicated voltage reference IC is not justified by the design requirements.
- Reading a battery voltage with a microcontroller by dividing it down to within the ADC input range, then scaling the reading back up in software.
- Level-shifting logic signals between two voltage domains in low-speed digital circuits.
Common Mistakes to Avoid
- Ignoring the load connected to the output — the voltage divider formula Vout = Vin × R2 / (R1 + R2) assumes no current is drawn from the output. If a load draws significant current, it acts as a parallel resistance with R2 and drops Vout below the calculated value.
- Using resistor values that are too high, creating a high-impedance output susceptible to noise — for most practical circuits, keep total resistance in the low kilohm range unless power consumption is a critical constraint.
- Swapping R1 and R2 in the formula — R1 is the resistor connected to Vin (the top resistor) and R2 is the one connected to ground (the bottom resistor). Swapping them gives the complementary voltage instead of the intended output.
- Using a voltage divider to power a load that draws variable current — voltage dividers are not voltage regulators. Any change in load current changes the output voltage. Use a regulator for power delivery.
Frequently Asked Questions
Can a voltage divider power a load?
Only very light loads. For substantial loads use a buffer or regulator.
How do I choose R1 and R2?
Pick the ratio R2/(R1+R2) for desired output. Keep total resistance appropriate for your application.
What if R1 and R2 are equal?
Output is exactly half the input voltage.
Learn More
Guide
How to Calculate Voltage Dividers
Learn how to design a resistive voltage divider using the voltage divider formula. Covers output voltage calculation, loading effects, and practical design guidelines.
Ready to run the numbers?
Open Voltage Divider Calculator