Voltage Divider Calculator and Guide
A voltage divider uses two resistors to produce an output voltage that is a fraction of the input voltage. It is one of the most frequently used circuits in electronics — appearing in sensor interfaces, biasing circuits, ADC inputs, and level shifters.
Open Voltage Divider SimulatorVoltage Divider Diagram
The Voltage Divider Formula
Connect R1 from the input voltage to the output node, and R2 from the output node to ground. The output voltage is:
Example 1: Vin = 12V, R1 = 10kΩ, R2 = 10kΩ → Vout = 12 × 10/(10+10) = 6V. Exactly half.
Example 2: Vin = 5V, R1 = 10kΩ, R2 = 4.7kΩ → Vout = 5 × 4.7/(10+4.7) = 1.6V.
Choosing Resistor Values
The resistor ratio determines the output voltage. The absolute values determine current draw and loading sensitivity.
- High values (100kΩ+): Very little current drawn from the supply. But sensitive to loading — any connected circuit with lower input impedance will shift the output voltage significantly.
- Low values (1kΩ–10kΩ): Stiff output, less affected by loading. But draws more current continuously, which matters in battery-powered circuits.
- Rule of thumb: Make the divider resistance 10× lower than the load impedance for less than 10% error from loading.
Practical Applications
- Sensor interfaces: Many sensors (NTC thermistors, LDRs, potentiometers) change resistance with their measured quantity. Pair them with a fixed resistor in a divider to convert resistance change to a voltage readable by an ADC.
- Level shifting: Convert a 5V logic signal to 3.3V for a microcontroller that cannot tolerate 5V inputs. Use R1 = 2kΩ and R2 = 3.3kΩ for Vout = 5 × 3.3/5.3 ≈ 3.1V.
- Transistor biasing: Set the base voltage of a BJT transistor in a common-emitter amplifier using a voltage divider to establish the DC operating point.
- Reference voltage generation: Create a stable intermediate reference voltage from a regulated supply for op-amp circuits.
Loaded vs Unloaded Divider
When you connect a load (any circuit drawing current) to the divider output, the effective R2 becomes the parallel combination of R2 and the load resistance. This lowers the output voltage.
To minimise this effect, use a buffer (op-amp voltage follower) between the divider output and the load. The buffer presents virtually infinite impedance to the divider while driving the load from its low-impedance output.
Common Beginner Mistakes
Ignoring Loading Effects
A voltage divider's output voltage changes when you connect a load. If the load resistance is comparable to R2, the output drops significantly. This is the most commonly overlooked issue in voltage divider design.
Divider Resistors Too High for Power Supply
Very high resistance values (MΩ range) make the divider extremely sensitive to leakage currents and input bias currents of connected circuits. Keep divider resistance 10× lower than the load impedance.
Using as a Power Supply Substitute
A voltage divider is not a voltage regulator. Under varying load, the output voltage varies. For any application requiring stable voltage under changing current demand, use a proper voltage regulator (LM7805, LM317, or switching regulator).
Wasting Power Continuously
A voltage divider draws current continuously — even when the load is off. In battery-powered circuits, choose R values high enough to minimise this quiescent current drain while still providing a stiff enough output for your load.