Last updated: 2026-06-15

Voltage divider

A voltage divider is the simplest way to obtain a smaller voltage from a larger one: two resistors in series, with the output taken from the junction between them. The output is a fixed fraction of the input set by the ratio of the resistors:

The same current flows through both resistors, $I = V_{in}/(R_1 + R_2)$. This calculator returns the output voltage and that series current.

How the ratio works

The input voltage is shared between the two resistors in proportion to their resistance. The output, taken across $R_2$, gets the share $R_2/(R_1+R_2)$. If the two resistors are equal the output is exactly half the input; making $R_2$ larger raises the output toward the full input, and making it smaller lowers it toward zero.

Worked example

With $V_{in} = 12\ \text{V}$, $R_1 = 1\ \text{k}\Omega$ and $R_2 = 2\ \text{k}\Omega$:

The series current is $I = 12 / 3000 = 4\ \text{mA}$.

Loading and resistor choice

The formula assumes nothing draws current from the output. A load connected across $R_2$ sits in parallel with it, lowers the effective lower resistance, and pulls the output below the calculated value. Keep the load resistance much larger than $R_2$ — at least ten times is a common guideline — or use a buffer or regulator instead. Only the ratio sets the output, so choose the overall resistance to balance power draw against sensitivity to loading and noise.

Where dividers are used

Voltage dividers set reference voltages, bias transistor stages, read the wiper of a potentiometer, and scale a high voltage down into the input range of an analogue-to-digital converter. They are a measurement and signalling tool, not a power supply: to drive a load that needs real current, a regulator is the right component.

Limitations

This is an ideal-resistor model with no load on the output. Real dividers are affected by load current, by resistor tolerance (which shifts the exact ratio), and by temperature. For precision references, use tight-tolerance resistors or a dedicated voltage-reference IC.