Calculate the missing voltage divider value—output voltage, input voltage, R1, or R2—from any three known voltages and resistances.
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Voltage Divider Formula
A voltage divider uses two resistors in series to create an output voltage that is a fraction of the input voltage. For the standard divider, R1 is the upper resistor connected to the input voltage, and R2 is the lower resistor connected to ground. The output voltage is taken across R2.
Vout = Vin * R2 / (R1 + R2)
To solve for input voltage:
Vin = Vout * (R1 + R2) / R2
To solve for R1:
R1 = R2 * (Vin - Vout) / Vout
To solve for R2:
R2 = R1 * Vout / (Vin - Vout)
- Vout = output voltage across R2
- Vin = input voltage applied across R1 and R2 together
- R1 = upper resistor in the divider
- R2 = lower resistor in the divider
The calculator lets you leave exactly one field blank. If you enter Vin, R1, and R2, it calculates Vout. If you enter Vout, R1, and R2, it calculates Vin. If you enter the two voltages and one resistor, it solves for the missing resistor. Voltage units are converted to volts internally, and resistance units are converted to ohms internally before the formula is applied.
Common Voltage and Resistance Unit Conversions
Use these conversions to check that values entered in different units are equivalent.
| Unit | Meaning | Base Unit Equivalent |
|---|---|---|
| 1 mV | 1 millivolt | 0.001 V |
| 1 V | 1 volt | 1 V |
| 1 kV | 1 kilovolt | 1000 V |
| 1 Ω | 1 ohm | 1 Ω |
| 1 kΩ | 1 kiloohm | 1000 Ω |
| 1 MΩ | 1 megaohm | 1,000,000 Ω |
Typical Divider Ratios
The ratio of R2 to the total resistance determines the output voltage fraction.
| R1 | R2 | Vout as Fraction of Vin | Example with 12 V Input |
|---|---|---|---|
| Equal to R2 | Equal to R1 | 1/2 Vin | 6 V |
| 2 × R2 | 1 × R2 | 1/3 Vin | 4 V |
| 3 × R2 | 1 × R2 | 1/4 Vin | 3 V |
| 1 × R2 | 2 × R1 | 2/3 Vin | 8 V |
Example Problems
Example 1: Find the output voltage
You have a 12 V input, R1 = 10 kΩ, and R2 = 5 kΩ.
Vout = 12 * 5000 / (10000 + 5000)
Vout = 4 V
The output voltage is 4 V.
Example 2: Find R2
You need 3.3 V output from a 5 V input, and R1 = 10 kΩ.
R2 = 10000 * 3.3 / (5 - 3.3)
R2 = 19411.76 ohms
R2 should be about 19.4 kΩ.
FAQ
Why does the output voltage depend on R2?
The output is measured across R2, so the voltage across R2 is the portion of the input voltage assigned to that resistor. A larger R2 gives a larger share of the total voltage. A smaller R2 gives a smaller share.
Can a voltage divider increase voltage?
No. A passive two-resistor voltage divider can only produce an output voltage between 0 and the input voltage. If you need a higher voltage than the input, you need a boost converter, transformer circuit, or another active circuit.
Why does the load connected to the output matter?
A real load acts like another resistance connected across R2. This changes the effective lower resistance and can reduce the output voltage. Voltage dividers work best when the load resistance is much larger than R2, or when the divider feeds a high-impedance input.