Voltage Divider Calculator | R1 R2 Output Voltage
Calculate output voltage from a resistor voltage divider (Vout = Vin × R2/(R1+R2)), or calculate R2 from desired output voltage.
How to Use
- To calculate output voltage: enter Vin, R1, and R2, then click 'Calculate Vout'.
- To find R2 for a target output: enter Vin, R1, and desired Vout, then click 'Calculate R2'.
- The formula is: Vout = Vin × R2 / (R1 + R2).
- The tool also shows current draw through the divider and power dissipation in each resistor.
- For accurate results, use the actual resistor values (including tolerance effects).
- Note: voltage divider output voltage changes when a load is connected — use a buffer amplifier for low-impedance loads.
About Voltage Dividers
How a Voltage Divider Works
A voltage divider consists of two series resistors (R1 and R2) connected between the supply voltage and ground. The output is taken from the junction between them. By Kirchhoff's Voltage Law and Ohm's Law: Vout = Vin × R2 / (R1 + R2). The output is always less than the input. The ratio R2/(R1+R2) determines what fraction of Vin appears at the output.
Loading Effect
When a load is connected to Vout, it forms a parallel combination with R2, reducing the effective lower resistance and causing Vout to drop. For minimal loading effect, the load resistance should be at least 10× the value of R2. If a stable output voltage is needed under varying loads, use a voltage regulator or an op-amp buffer (voltage follower) after the divider.
Practical Design Considerations
Choose resistor values that provide the desired Vout ratio while keeping current draw acceptable. Very high resistance values (MΩ) are prone to noise pickup; very low values waste power. A divider current of 1/10 the expected load current provides a reasonable trade-off. Always account for resistor tolerance — a 5% tolerance can shift Vout by up to ±10% in a worst-case scenario.
Applications in Sensor Interfacing
Voltage dividers are widely used to interface sensors with microcontrollers. For example, a thermistor + fixed resistor divider produces a voltage that varies with temperature. Strain gauges, light-dependent resistors (LDRs), and potentiometers are all used in voltage divider configurations. Level shifters use dividers to convert 5V logic signals down to 3.3V for modern microcontrollers.
Key Features
- Calculates Vout from Vin, R1, and R2
- Reverse calculates R2 for a target Vout
- Shows current draw and power dissipation per resistor
- Handles wide resistance ranges (Ω to MΩ)
Common Applications
- Level shifting 5V signals to 3.3V for microcontroller inputs
- Setting reference voltages for comparators and op-amps
- Interfacing variable resistance sensors (thermistors, LDRs)
- Biasing transistor base circuits
- Battery voltage monitoring with ADC inputs