Voltage Divider Calculator

A voltage divider is two resistors in series that turn a higher voltage into a lower one. The input voltage (Vin) goes across both resistors, R1 and R2, and the output (Vout) is taken across R2:

Vout = Vin * R2 / (R1 + R2)

Enter any three of the four, and the calculator finds the fourth. The optional load resistance (RL), the resistance of whatever is connected to the output, makes it take the load into account, and it also shows the power used.

Example without a load

12 V divided by R1 = 10 kΩ and R2 = 4.7 kΩ:

Vout = 12 * 4,700 / (10,000 + 4,700) = 3.837 V

Choosing the resistors

Pick R1, then: R2 = R1 * Vout / (Vin - Vout). To bring 5 V down to 3.3 V with R1 = 1 kΩ:

R2 = 1,000 * 3.3 / (5 - 3.3) = 1,941 Ω, so the nearest standard value, 2 kΩ, would do.

With a load

A load connected to the output is in parallel with R2, which lowers the resistance at the bottom of the divider, and so the output voltage:

R2∥RL = R2 * RL / (R2 + RL)

Vout = Vin * (R2∥RL) / (R1 + R2∥RL)

In the example above, a 10 kΩ load gives R2∥RL = 3,197 Ω, and the output drops from 3.84 V to 2.907 V.

To keep the effect small, make the load at least ten times R2, or use smaller resistors, which waste more power. The calculator shows that power:

• Total power: Ptot = (Vin - Vout)² / R1 + Vout² / R2 + Vout² / RL, 10.9 mW here.
• Load power: PRL = Vout² / RL, 0.845 mW here.

Frequently asked questions

Can a voltage divider power a device?

Only a very light one, such as the input of a microcontroller or an ADC, or a sensor reference. As soon as the device draws current, the output voltage drops, and it changes whenever the current does. For power, use a voltage regulator, such as the LM317 (LM317 Calculator).

What is a potentiometer?

A voltage divider in one part: a resistor with a sliding contact, which splits it into R1 and R2. Turning it changes the ratio, as in a volume knob.