A voltage divider uses two resistors to turn a higher voltage into a lower one. It is useful for sensor signals, reference voltages, and measuring a voltage that is too high for an input pin. The calculator can find the output voltage, source voltage, R1, or R2 when the other three values are known.
Source voltage (Vs)
│
R1
│──── Output voltage (Vout)
R2
│
GND
How to Calculate Voltage Divider Output and Resistor Values (step by step)
Step One: Identify the resistor positions
R1 is the resistor between the source and the output node. R2 is the resistor between the output node and ground. The output voltage is measured across R2.
R1 = resistor connected to Vs
R2 = resistor connected to GND
For the default values, the source is 12 V, R1 is 10000 Ω, and R2 is 10000 Ω.
Step Two: Calculate the output voltage
Use the voltage divider relationship to calculate the voltage at the junction between the resistors.
Output voltage = Source voltage × R2 ÷ (R1 + R2)
Vout = 12 × 10000 ÷ (10000 + 10000)
Vout = 6 V
When R1 and R2 have equal resistance, the output is half of the source voltage.
Step Three: Solve for the source voltage
If the output voltage and both resistors are known, rearrange the formula to find the source.
Source voltage = Output voltage × (R1 + R2) ÷ R2
Vs = 6 × (10000 + 10000) ÷ 10000
Vs = 12 V
Step Four: Solve for R1
To find the upper resistor, use the source-to-output ratio and multiply it by R2.
R1 = R2 × (Source voltage ÷ Output voltage − 1)
R1 = 10000 × (12 ÷ 6 − 1)
R1 = 10000 Ω
Step Five: Solve for R2
To find the lower resistor, multiply R1 by the output voltage and divide by the difference between source and output voltage.
R2 = R1 × Output voltage ÷ (Source voltage − Output voltage)
R2 = 10000 × 6 ÷ (12 − 6)
R2 = 10000 Ω
What your results mean
A lower R2 produces a lower output voltage because more voltage is dropped across R1. A higher R2 moves the output closer to the source voltage. The resistor values also control current through the divider, so extremely high values reduce wasted current while making the circuit more sensitive to loading.
This calculator treats the output as unloaded. If another circuit draws meaningful current from the output node, include that load as a parallel resistance with R2 before calculating. For a load resistance of RL, the effective lower resistance is R2 in parallel with RL, not R2 alone.
Use the same resistance units for comparison, but the calculator can convert ohms, kilohms, and megohms automatically. Avoid setting the source and output voltages equal when solving for a resistor, because the rearranged equation would require division by zero.