What is the voltage divider formula?

Vout = Vin × R2 / (R1 + R2). R1 is the top resistor (between Vin and Vout) and R2 is the bottom resistor (between Vout and GND).

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

A load in parallel with R2 lowers the effective resistance, which reduces Vout. For a stiff divider, keep the load resistance at least 10× larger than R2.

How do I choose R1 and R2?

First fix the ratio R2/(R1+R2) to get the desired Vout/Vin fraction. Then pick absolute values high enough to limit current (and power), but low enough to be unaffected by leakage or load impedance.

What units should I use?

You can enter resistances in ohms (Ω), kilohms (kΩ), or megaohms (MΩ) using the drop-down next to each field. Voltage is always in volts.

Is this calculator accurate for AC circuits?

This tool assumes pure resistors at DC or low frequency. For AC circuits with capacitors or inductors, impedances are complex (frequency-dependent) and require a different analysis.

Voltage Divider Calculator

Enter input voltage and resistor values to find output voltage, current draw, and power dissipation for a resistive voltage divider.

Circuit Inputs

Input Voltage (Vin)

R1 (top resistor)

R2 (bottom resistor)

Output Voltage (Vout)

4.000 V

Ratio: 0.333

Current

800.0 µA

Through R1 + R2

Total Power

9.600 mW

Dissipated by R1 + R2

Power in R1

6.400 mW

I² × R1

Power in R2

3.200 mW

I² × R2

Solve for a Resistor

Know the desired Vout? Find R1 or R2 given the other resistor.

Desired Vout V

Fix

Summary

Enter input voltage and resistor values to find output voltage, current draw, and power dissipation for a resistive voltage divider.

How it works

Enter the input voltage (Vin) in volts.
Enter the values for R1 (top resistor) and R2 (bottom resistor) in ohms.
The output voltage Vout is taken across R2.
Results update instantly — no button needed.
Use the unit selector to enter resistances in ohms, kilohms, or megaohms.

Use cases

Level-shift a 5V signal down to 3.3V for microcontroller GPIO inputs.
Set a reference voltage for an op-amp or comparator circuit.
Calculate the bias resistors for a transistor base network.
Choose resistor values that keep power dissipation within component ratings.
Verify a sensor voltage divider outputs the correct ADC input range.
Design an audio volume control or attenuator network.

Frequently Asked Questions

Last updated: 2026-09-22 · Reviewed by Nham Vu