Voltage Divider Calculator
Find the output voltage of a two-resistor voltage divider.
Formula
Vout = Vin × R2 / (R1 + R2)
Inputs
- Input voltage (Vin) (V)
- R1 (top resistor) (Ω)
- R2 (bottom resistor) (Ω)
What it does
Two resistors in series split an input voltage. The output is taken across the bottom resistor: Vout = Vin × R2 / (R1 + R2).
Where it's used
Scaling a voltage into a microcontroller’s ADC range, setting regulator feedback, and biasing transistors.
How to use it
- Enter the input voltage.
- Enter R1 (between Vin and the output) and R2 (between the output and ground).
- Read the output voltage.
The formula explained
- Vin
- voltage across the whole divider
- R1
- top resistor
- R2
- bottom resistor
- Vout
- voltage across R2
Worked example
Scaling a 12 V signal down for a 3.3 V ADC.
Inputs:
- Input voltage (Vin): 12 V
- R1 (top resistor): 10k Ω
- R2 (bottom resistor): 3.3k Ω
The calculator gives:
- Result: 2.9774 V
Tips and common mistakes
- The result assumes nothing draws current from Vout. A load acts in parallel with R2, so keep it at least 10× larger than R2.
- A divider is not a power supply — use a regulator to power circuits.
- Larger resistors waste less current but pick up more noise.
Questions people ask
How do I choose R1 and R2 for a voltage I want?
Pick R2, then R1 = R2 × (Vin / Vout − 1). To get 3.3 V from 5 V with R2 = 20 kΩ, R1 works out at about 10.3 kΩ. A standard 10 kΩ gives 3.33 V, close enough for most uses.
How much current does a divider waste?
The current through both resistors is Vin / (R1 + R2). A 10 kΩ + 3.3 kΩ divider on 12 V draws about 0.9 mA all the time. On battery power use larger values, as long as the load stays much larger than R2.
Can I read a 5 V signal on a 3.3 V microcontroller with a divider?
For slow signals, yes: R1 = 1 kΩ on top and R2 = 2 kΩ below give 3.33 V. For fast signals such as SPI clocks, the resistors and pin capacitance round off the edges, so use a level-shifter chip instead.
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Enter your values to get the result with the formula, substituted values and an explanation of each step.