Voltage Divider Calculator
Find Vout, R1, or R2 for a resistive voltage divider.
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How Voltage Dividers Work
A voltage divider uses two resistors in series to produce an output voltage that is a fraction of the input voltage. The formula is Vout = Vin × R2 / (R1 + R2).
Voltage dividers are used in sensor circuits, ADC reference inputs, level shifters, and biasing circuits. They work best when the load impedance is much higher than R2.
For a 50/50 split, use equal resistors. For a 1/3 split, use R1 = 2 × R2. Common values range from 1kΩ to 100kΩ depending on power constraints.
A voltage divider uses two resistors in series to produce an output voltage lower than the supply. The formula: V_out = V_in × R2 / (R1 + R2). R2 is the resistor connected to ground; R1 is connected to V_in.
Example: scaling a 12V sensor output to 3.3V for an Arduino input. Set R1 = 10kΩ and R2 = 3.9kΩ: V_out = 12 × 3.9 / (10 + 3.9) = 12 × 0.281 = 3.37V. Close enough; fine-tune R2 if precision is critical.
Voltage dividers work best when the load impedance is at least 10× the parallel combination of R1 and R2. A low-impedance load "steals" current through R2, pulling V_out down. For ADC inputs (typically high-impedance), this is rarely an issue.
Avoid using voltage dividers to power loads that draw significant current — use a proper voltage regulator instead. A voltage divider is efficient only when the load current is negligible compared to the divider current.
See also: Ohm's Law Calculator · RC Time Constant Calculator