Voltage Divider Calculator

What is a voltage divider?

Two resistors in series share the applied voltage in proportion to their values. The voltage across the lower resistor is a fixed fraction of the input. Dividers are used to read sensors, make reference voltages and scale a signal down to a lower voltage input.

Formula

V_out = V_in × R2 / (R1 + R2)

R1 connects to the input and R2 to ground. The output is taken across R2. For a wanted output, R2 = R1 × V_out / (V_in − V_out) and R1 = R2 × (V_in − V_out) / V_out.

Example

With 12 V in, R1 = 10 kΩ and R2 = 5 kΩ the output is 12 × 5 / 15 = 4 V. The divider carries 0.8 mA and dissipates 9.6 mW.

Effect of a load

A load on the output sits in parallel with R2 and lowers the resistance of the lower leg. Put 10 kΩ on the divider above and the lower leg becomes 3.33 kΩ, so the output is 3 V instead of 4 V. The drop stays small when the load is at least ten times R2. That is why a divider can only feed inputs that draw very little current.

Standard resistors

The calculated value is rarely a stock part. The tool gives the nearest value in the IEC 60063 E12 or E24 series and the real output voltage with it.

Limits

A divider does not hold the voltage steady: the output follows the input and the load. Use a regulator to power a circuit. Resistor tolerances shift the output too.