Voltage Divider Calculator and Formula

This interactive voltage divider calculator allows you to obtain the output voltage ( $V_{out}$ ) of a passive voltage divider circuit immediately. Below, we explain the physical and mathematical theory behind this fundamental circuit.

What is a Voltage Divider?

A voltage divider (or potential divider) is a simple passive linear circuit that reduces an input voltage ( $V_{in}$ ) to a fraction of it as output voltage ( $V_{out}$ ).

It typically consists of two resistors in series ( $R_1$ and $R_2$ ) connected to an input power supply. Voltage dividers are widely used in electronics to:

Adapt signal levels for microcontrollers (such as ADC pins on Arduino or ESP32).
Bias active components like BJT transistors and operational amplifiers.
Create stable reference voltages.

Voltage Divider Formula

The physical principle of a voltage divider is based on Ohm's Law and Kirchhoff's voltage laws. Since the same current flows through both resistors in series, the general formula for calculating the output voltage is:

$V_{out} = V_{in} \times \frac{R_2}{R_1 + R_2}$

Where:

$V_{out}$ (Output Voltage): The voltage obtained at the midpoint between the two resistors, expressed in volts ( $V$ ).
$V_{in}$ (Input Voltage): The potential difference of the power source, in volts ( $V$ ).
$R_1$ (First resistor): The resistor connected to the input voltage, in ohms ( $\Omega$ ).
$R_2$ (Second resistor): The resistor connected to ground (GND), in ohms ( $\Omega$ ).

Practical Calculation Example

Imagine you have an analog sensor that delivers a signal of $5\,\text{V}$ ( $V_{in}$ ), but you want to read it safely on a microcontroller that only supports a maximum voltage of $3.3\,\text{V}$ (like the ESP32).

For this, you can use our voltage divider calculator by choosing commercial values for the resistors:

We define $R_1 = 10\,\text{k}\Omega$ ( $10000\,\Omega$ ).
We define $R_2 = 20\,\text{k}\Omega$ ( $20000\,\Omega$ ).
We apply the formula: $V_{out} = 5\,\text{V} \times \frac{20000}{10000 + 20000} = 5 \times \frac{20000}{30000} = 3.33\,\text{V}$

This value is ideal to protect the analog input of the microcontroller.

Frequently Asked Questions about Voltage Dividers

How does the load connected to the voltage divider affect it?

When you connect a load (or load resistance $R_L$ ) in parallel with $R_2$ to read the signal, the equivalent resistance changes to $R_{eq} = R_2 \parallel R_L$ . This alters the voltage ratio and reduces the actual output voltage. To avoid this effect, the input resistance of the connected device must be much larger than $R_2$ (at least 10 times larger).

Can I use a voltage divider to power a motor or a powerful LED?

No. Resistive voltage dividers are not efficient for powering power loads because they dissipate too much energy as heat and the voltage drops quickly under variations in current consumption. To power devices, you should use linear voltage regulators (like the LM78xx series) or switching regulators (Buck Converters).