Voltage Divider Calculator
Find Vout for any R1/R2 combination, or solve backwards for a target output voltage.
Vout
3.837 V
Current
816 µA
R1 / R2 power
6.66 mW / 3.13 mW
Formula
Vout = Vin × R2 / (R1 + R2)
Vout is the voltage across R2, the lower resistor.
How a voltage divider works
Two resistors in series share the input voltage in proportion to their values. The output, tapped between them, is a fixed fraction of the input set purely by the R1:R2 ratio — shown live in the schematic above.
Switch to 'Find R2' to work backwards: given a target output, the tool solves for the resistor you need.
Important limitation
- A divider is for sensing or reference voltages, not for powering a load — drawing current changes the output.
- Keep the load resistance at least 10× larger than R2, or the divider 'sags' under load.
- Lower resistor values give a stiffer output but waste more current as heat; higher values save power but are noise-prone.
Where it's used
Scaling a 12 V battery down to the 3.3 V an ADC can read, setting reference thresholds, and reading potentiometers and sensors like LDRs and thermistors.
Frequently Asked Questions
What is a voltage divider?+
Two resistors in series connected to a supply voltage. The output is tapped between them: Vout = Vin × R2 / (R1 + R2).
When should I use a voltage divider?+
Common uses: level-shifting a 5 V signal to 3.3 V for a microcontroller pin, biasing a transistor, or creating a reference voltage. Avoid high-current loads — use a dedicated regulator instead.