Voltage Divider Calculator
Find the output voltage of a two-resistor voltage divider circuit.
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Formulas
Vout = Vin × R2 / (R1 + R2)Vin = Input voltage, R1 = Top resistor, R2 = Bottom resistor, Vout = Output voltage
How a Voltage Divider Works
A voltage divider turns a large voltage into a smaller one using two resistors in series. Used in sensor circuits, level shifting, biasing transistors, and ADC reference voltages.
Important limitation
Only works accurately when load impedance is much higher than R2 (at least 10x). For power applications, use a voltage regulator instead.
How a Voltage Divider Works
A voltage divider uses two series resistors to produce an output voltage that is a fraction of the input. It is one of the most common building blocks in electronics, used for setting reference voltages, scaling signals, and reading sensors.
Vout = Vin × R2 / (R1 + R2)Vin is the input voltage, R1 is the top resistor, and R2 is the bottom resistor across which the output is measured.
The output is always a fraction of the input, set purely by the ratio of R2 to the total resistance. Equal resistors give exactly half the input.
Worked Examples
Vin = 9 V, R1 = R2 = 10 kΩ:
Vout = 9 × 10000 / (10000 + 10000) = 4.5 V
To drop 5 V to ~3.3 V, use R1 = 1.7 kΩ, R2 = 3.3 kΩ:
Vout = 5 × 3300 / (1700 + 3300) = 3.3 V
The Loading Effect
A voltage divider only holds its calculated value when little or no current is drawn from the output. Connecting a load in parallel with R2 effectively lowers R2 and reduces Vout. As a rule, keep the load resistance at least 10× larger than R2, or use an op-amp buffer for a stable output. This is why dividers are ideal for high-impedance inputs like an ADC pin but unsuitable for powering a load directly.
Frequently Asked Questions
Can I power a circuit from a voltage divider?
Not reliably. Dividers are for reference or signal voltages, not power delivery. Any meaningful current draw collapses the output. Use a voltage regulator instead.
How do I pick resistor values?
Choose the ratio for the voltage you want, then scale both resistors up to limit current and waste. Values in the kΩ to tens-of-kΩ range balance power loss against loading.
What about a potentiometer?
A potentiometer is an adjustable voltage divider — the wiper position sets the R1:R2 ratio continuously.