Potential Divider Calculator

Calculate output voltage and resistor values for voltage divider circuits

Enter any three values to calculate the fourth. Leave the unknown value blank.

Input Voltage (Vin) Volts (V)

Output Voltage (Vout) Volts (V)

Resistor 1 (R1) Ohms (Ω)

Resistor 2 (R2) Ohms (Ω)

What Is a Potential Divider?

A potential divider, also known as a voltage divider, is a fundamental circuit configuration that produces an output voltage that is a fraction of its input voltage. The circuit consists of two resistors connected in series between a voltage source and ground, with the output voltage measured at the junction between the two resistors.

This simple yet powerful circuit is one of the most commonly used configurations in electronics, found in everything from sensor interfaces to signal conditioning circuits. The potential divider allows you to create a reference voltage or scale down a voltage to a level suitable for measurement or processing by electronic components.

The Potential Divider Formula

The output voltage of a potential divider is determined by the ratio of the resistances and the input voltage. The fundamental formula is:

Vout = Vin × R2 / (R1 + R2)

Where:

Vout is the output voltage (measured across R2)
Vin is the input voltage applied across the entire circuit
R1 is the resistance of the upper resistor (connected to Vin)
R2 is the resistance of the lower resistor (connected to ground)

The current flowing through both resistors can be calculated as:

I = Vin / (R1 + R2)

How Does a Potential Divider Work?

The operation of a potential divider is based on Ohm’s law and the principle that current is constant in a series circuit. When a voltage is applied across two resistors in series, the same current flows through both resistors. Each resistor develops a voltage drop proportional to its resistance.

Key Principles

The total voltage drop across both resistors equals the input voltage
The voltage across each resistor is proportional to its resistance
A larger resistance receives a larger share of the input voltage
When R1 equals R2, the output voltage is exactly half the input voltage
The output voltage is always less than or equal to the input voltage

The voltage division ratio is determined solely by the ratio of resistances, not their absolute values. For example, a 1kΩ and 2kΩ divider produces the same output voltage ratio as a 10kΩ and 20kΩ divider, though the current draw differs.

Practical Examples

Example 1: 12V to 5V Conversion

Scenario: You need to reduce a 12V supply to 5V for a microcontroller analogue input.

Given: Vin = 12V, Vout = 5V, R1 = 4.7kΩ

Calculation:
5 = 12 × R2 / (4700 + R2)
5(4700 + R2) = 12 × R2
23500 = 7R2
R2 = 3357Ω (use 3.3kΩ standard value)

Result: With R1 = 4.7kΩ and R2 = 3.3kΩ, the actual output is 4.95V, which is within acceptable tolerance for most applications.

Example 2: Half-Voltage Reference

Scenario: Create a 4.5V reference from a 9V battery.

Given: Vin = 9V, Vout = 4.5V

Solution: Since the output is exactly half the input, use equal resistors. Common choices are 10kΩ for each (higher values reduce current consumption).

Current draw: I = 9V / 20kΩ = 0.45mA (very low power consumption)

Example 3: Sensor Interface

Scenario: Scale a 0-10V sensor output to 0-3.3V for an ADC input.

Given: Vin = 10V, Vout = 3.3V, R2 = 10kΩ

Calculation:
3.3 = 10 × 10000 / (R1 + 10000)
3.3(R1 + 10000) = 100000
R1 = 20303Ω (use 20kΩ standard value)

Result: This configuration provides the voltage scaling needed to safely interface the sensor with a 3.3V tolerant input.

Common Applications

Voltage Reference Generation

Potential dividers create stable reference voltages for comparators, analogue-to-digital converters, and operational amplifier circuits. They’re particularly useful when you need a simple, low-cost voltage reference that doesn’t require precision regulation.

Signal Conditioning

Many sensors produce output voltages that exceed the input range of measurement devices. A potential divider scales these signals to appropriate levels whilst maintaining the signal’s proportional characteristics.

Biasing Circuits

Transistor and operational amplifier circuits often require specific bias voltages. Potential dividers provide these voltages from a single supply rail, simplifying circuit design and reducing component count.

Level Shifting

When interfacing between different logic voltage levels (e.g., 5V to 3.3V), potential dividers provide a simple solution for level translation, particularly for digital signals that don’t require fast switching speeds.

Battery Voltage Monitoring

In battery-powered devices, potential dividers allow microcontrollers to monitor battery voltage by scaling it down to a measurable range, enabling low-battery warnings and power management features.

Design Considerations

Resistor Selection

Choose resistor values based on a balance between current consumption and loading effects. Higher resistance values reduce current draw but make the circuit more susceptible to loading and noise. For most applications, total resistance between 10kΩ and 100kΩ provides a good compromise.

Loading Effects

The output of a potential divider has a source impedance equal to the parallel combination of R1 and R2. When connected to a load, this impedance can affect the output voltage. To minimise loading effects, the load impedance should be at least 10 times greater than the output impedance of the divider.

Power Dissipation

Current flows continuously through a potential divider, dissipating power as heat. Calculate the power dissipation for each resistor using P = I²R, and ensure resistors are rated appropriately. For battery-powered applications, consider using higher resistance values to reduce power consumption.

Tolerance and Accuracy

The accuracy of the output voltage depends on resistor tolerances. Standard 5% resistors may be sufficient for non-critical applications, but precision work requires 1% or better tolerance components. The actual output voltage tolerance is roughly the sum of the individual resistor tolerances.

Temperature Effects

Resistor values change with temperature. If both resistors have the same temperature coefficient, their ratio remains relatively constant, minimising output voltage drift. Use resistors from the same batch or type to maintain ratio stability.

Frequently Asked Questions

Can I use a potential divider to power a circuit?

Potential dividers are generally not suitable for powering circuits because they cannot supply significant current without the output voltage dropping substantially. The output voltage changes when current is drawn because of the voltage drop across R1. For powering circuits, use a voltage regulator instead.

Why does my output voltage change when I connect a load?

This is due to loading effects. The load draws current, creating an additional voltage drop across R1 and reducing the output voltage. To minimise this effect, either reduce the divider resistances, increase the load impedance, or buffer the output with an op-amp or voltage follower.

What resistor values should I choose?

For general-purpose applications, total resistance between 10kΩ and 100kΩ works well. Use lower values (1kΩ-10kΩ) when driving loads or in noisy environments. Use higher values (100kΩ-1MΩ) for low-power battery applications where current consumption must be minimised.

Can I use a potential divider with AC voltages?

Yes, potential dividers work with AC voltages following the same principles. However, at higher frequencies, parasitic capacitance can affect performance. For AC applications, consider the frequency response and use resistors with low parasitic capacitance when necessary.

How do I account for resistor tolerance in my design?

Calculate the worst-case output voltage by considering both resistors at their tolerance extremes. For example, with 5% resistors, R1 could be 5% high whilst R2 is 5% low (or vice versa), giving approximately 10% variation in the output voltage. Use precision resistors for critical applications.

What’s the difference between a potential divider and voltage regulator?

A potential divider produces an output voltage that’s a fixed fraction of the input, but this voltage drops when current is drawn. A voltage regulator maintains a constant output voltage regardless of load current (within its specifications). Regulators are essential for powering circuits, whilst dividers are ideal for voltage references and signal conditioning.

Can I use variable resistors in a potential divider?

Yes, using a potentiometer as R2 (or both resistors) creates an adjustable voltage divider. This is common in volume controls, brightness adjusters, and sensor circuits. The potentiometer acts as both R1 and R2, with the wiper providing the adjustable output voltage.

Tips for Accurate Results

Measure actual resistor values with a multimeter rather than relying on colour codes, especially for precision applications
Use 1% tolerance resistors for applications requiring better than 5% accuracy
Keep resistor power dissipation below 50% of the rated value to prevent overheating and value drift
When prototyping, measure the actual output voltage and adjust resistor values as needed
Consider using a voltage follower buffer when the divider output drives a load
For battery monitoring, factor in the voltage drop across the divider when the battery is under load
Use metal film resistors for better temperature stability compared to carbon film types
In high-frequency applications, keep resistor leads short to minimise parasitic inductance