Voltage Divider Calculator

Voltage Divider Calculator

Circuit Diagram:
Vin ──┬──[R1]──┬──[R2]── GND
              │
              Vout

Voltage Divider Calculator — R1, R2, and Load

A voltage divider is one of the simplest and most useful circuits in electronics. Using just two resistors in series, it produces an output voltage that is a fraction of the input voltage. This calculator computes the output voltage, circuit current, and power dissipation, and optionally shows the effect of connecting a load resistor across the output.

The Voltage Divider Formula

The output voltage of an unloaded resistor divider is: Vout = Vin × R2 / (R1 + R2). The output is taken across R2. If R1 equals R2, the output is exactly half the input. If R2 is much larger than R1, the output approaches the input voltage.

The Loading Effect

In practice, when you connect a load (such as an integrated circuit, a sensor, or another circuit) across Vout, the load resistance RL appears in parallel with R2. This reduces the effective resistance and lowers the output voltage — a phenomenon called the loading effect. The calculator shows both unloaded and loaded output voltages so you can assess whether your divider will behave as intended in the real circuit.

Common Applications

  • Level shifting: Scale a 5 V signal to 3.3 V for microcontroller inputs.
  • Sensor biasing: Set a reference voltage for thermistors, LDRs, or potentiometers.
  • Battery monitoring: Scale a battery voltage into the ADC range of a microcontroller.
  • Volume/gain control: Adjust audio signal levels.

Design Tips

For a stiff divider (one that maintains its output voltage under load), the parallel combination of R2 and RL should be much greater than R1. A common rule of thumb is to keep R1 and R2 at least 10× smaller than the minimum expected load resistance. However, lower resistor values draw more quiescent current and waste more power.

Frequently Asked Questions

Can I use a voltage divider as a power supply?
Not for any significant current. Voltage dividers are suitable for signal-level voltage references only. For a power supply, use a voltage regulator IC (such as the 7805 or LM317) which maintains a stable output voltage regardless of load current.
What is a potentiometer?
A potentiometer is an adjustable voltage divider. It has three terminals: two ends connected across the input voltage and a wiper that slides along the resistive element, producing any fraction of the input voltage from 0% to 100%.
How do I choose R1 and R2 values?
Choose R2/(R1+R2) equal to the desired voltage ratio. Then pick an absolute scale (e.g., 10 kΩ total) that balances power dissipation and loading effect. Lower total resistance wastes more power; higher total resistance is more susceptible to loading.
Does the voltage divider formula work for AC?
Yes, for purely resistive AC circuits. For reactive components (capacitors, inductors), the formula still applies but uses impedances (complex numbers) instead of simple resistances, and the output will be frequency-dependent.