Voltage DividerFree online science calculator — instant results, no sign-up
Calculate the output voltage of a two-resistor voltage divider.
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About the Voltage Divider
The Voltage Divider calculator finds the output voltage at the junction of two resistors connected in series across an input voltage. R1 is the top resistor connected to the input, and R2 is the bottom resistor connected to ground; the output is measured across R2. With the default input of 12 V, R1 of 1000 Ω, and R2 of 2000 Ω, the calculated output is 8 V.
- Input voltage (V)
- The voltage applied across the complete resistor pair, measured in volts (V), with a default of 12 V.
- R1 (top) (Ω)
- The resistance between the input voltage and the output junction, measured in ohms (Ω), with a default of 1000 Ω.
- R2 (bottom) (Ω)
- The resistance between the output junction and ground, measured in ohms (Ω), with a default of 2000 Ω.
Formula and method
Vout = Vin × R2 / (R1 + R2); Ratio = R2 / (R1 + R2)
- VoutOutput voltage across R2, measured relative to ground in volts (V).
- VinInput voltage across the series-connected resistor pair, in volts (V).
- R1Top resistor between the input and output junction, in ohms (Ω).
- R2Bottom resistor between the output junction and ground, in ohms (Ω).
- RatioDimensionless fraction of the input voltage appearing across R2.
Add R1 and R2, divide R2 by that total to obtain the divider ratio, and multiply the unrounded ratio by Vin. For the default values, the total resistance is 1000 + 2000 = 3000 Ω, the ratio is 2000 / 3000 = 2/3, displayed as 0.6667, and Vout = 12 × 2000 / (1000 + 2000) = 8 V.
Worked example
These are the calculator's starting values and the results it actually produces for them.
Inputs
- Input voltage12 V
- R1 (top)1000 Ω
- R2 (bottom)2000 Ω
Results
- Output voltage8 V
- Ratio0.6667
- FormulaVout = Vin × R2 / (R1 + R2)
How to use the Voltage Divider
- 1Identify the output junction between R1 and R2, with R2 connected to ground.
- 2Enter the input voltage in volts; use 12 for the default example.
- 3Enter R1 (top) in ohms; use 1000 for the default example.
- 4Enter R2 (bottom) in ohms; use 2000 for the default example.
- 5Read the results: output voltage 8 V, ratio 0.6667, and the displayed voltage-divider formula.
Reading your result
The Output voltage row gives the voltage from the resistor junction to ground: 8 V for the default inputs. The Ratio row gives the fraction of Vin present at the output: 0.6667 is the rounded form of two-thirds. The Formula row shows Vout = Vin × R2 / (R1 + R2), confirming that the output is taken across R2 rather than R1.
Limitations
This calculation assumes ideal resistors, an ideal input voltage source, and no load drawing current from the output junction. A connected load effectively changes the bottom resistance and generally lowers the output voltage. Resistor tolerances, temperature changes, source resistance, and resistor power ratings are not included. Use nonnegative resistance values with R1 + R2 greater than zero; two zero resistances make the formula undefined. A resistor divider is not a regulated power supply.
Common questions
Why does R2 appear in the numerator rather than R1?+
The output is measured across the bottom resistor R2, so its share of the total resistance determines the output voltage. With R1 = 1000 Ω and R2 = 2000 Ω, R2 receives two-thirds of the 12 V input, giving 8 V.
Why does multiplying 12 by the displayed ratio 0.6667 not give exactly 8?+
The ratio is rounded for display. The calculation uses 2000 / 3000, or exactly two-thirds, rather than the rounded value 0.6667, so the output is 8 V.
Will the output remain 8 V when I connect a load?+
Not necessarily. A resistive load connected from the output to ground is in parallel with R2, reducing the effective bottom resistance and lowering the output. The unloaded result is a good approximation only when the load resistance is much larger than R2.
Does scaling both resistors by the same factor change the output?+
Under the unloaded ideal model, no: R2 / (R1 + R2) stays the same. Higher resistance values reduce divider current but make the output more sensitive to loading; lower values increase current and power consumption.
Does the calculator save my information?+
Calculations run in your browser. While signed in, inputs and results are automatically recorded in account history; saving a result creates a separate saved record.
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Results are estimates for informational purposes and are not financial, medical or legal advice.