Voltage Divider Calculator
Two resistors in series split an input voltage in a fixed, predictable ratio — the standard trick for stepping a voltage down for a sensor or logic input without a full regulator circuit.
Inputs
Result
8 V
Current through divider 4 mA
How it works
Vout = Vin × R2 ÷ (R1 + R2)
How the voltage divider calculator works
Vout = Vin × R2 ÷ (R1 + R2), where R2 is the resistor across which the output is measured.
Current through the divider is the same through both resistors in series: I = Vin ÷ (R1 + R2).
Worked example: 12 V in, R1 = 1000 Ω, R2 = 2000 Ω
- Vout = 12 × 2000 ÷ (1000 + 2000) = 24,000 ÷ 3,000 = 8 V.
- Current through the divider = 12 ÷ 3,000 = 0.004 A = 4 mA.
Common mistakes to avoid
Swapping which resistor is R1 and which is R2
Vout is measured across R2 specifically in this formula. Swap the two resistor values and you get a completely different output voltage — R1 = 2000, R2 = 1000 with the same 12 V gives 4 V, not 8 V.
Ignoring the load connected to the output
This formula assumes nothing else draws current from the Vout node. A real load in parallel with R2 pulls the actual output voltage lower than this ideal calculation, sometimes significantly if the load resistance is close to R2's.
Frequently asked questions
Why use a voltage divider instead of a voltage regulator?
Simplicity and cost for low-current, non-critical signals — like scaling a sensor's output down to fit a microcontroller's input range. For anything needing stable voltage under varying load, a regulator is the correct choice instead.
How much current does the divider itself waste?
Continuously, Vin ÷ (R1+R2) worth of current flows through the divider whether or not anything uses the output — larger resistor values waste less power but make the divider more sensitive to loading effects.
Can I use this to step a voltage up instead of down?
No — a resistive divider can only produce an output between 0 and Vin, never higher. Stepping up needs a different circuit entirely, like a boost converter.
What happens if R1 is zero?
Vout becomes equal to Vin — with no first resistor, all of the input voltage appears directly at the output.
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