🔌 Voltage Divider Calculator
Compute the output voltage (Vout), current and power from two resistors and an input voltage. Everything runs in your browser — no data leaves this page.
What is a Voltage Divider?
A voltage divider is a simple but extremely common circuit that uses two series resistors to scale an input voltage down to a smaller output voltage. The output is taken across the second resistor (R2) and is given by the formula Vout = Vin × R2 / (R1 + R2). With this voltage divider calculator you enter Vin, R1 and R2 and immediately see the output voltage, the current flowing through the divider, the power dissipated in the resistors and the divider ratio. Every calculation happens in your browser; no data is ever sent to a server.
Vout formula and a worked example
Suppose Vin = 5 V, R1 = 1 kΩ and R2 = 2 kΩ. Then Vout = 5 × 2000 / (1000 + 2000) = 5 × 0.667 ≈ 3.33 V. The current is I = Vin / (R1 + R2) = 5 / 3000 ≈ 1.67 mA and the total power dissipated is P = Vin² / (R1 + R2) ≈ 8.3 mW. Because the ratio R2 / (R1 + R2) = 0.667, scaling R1 and R2 up or down together leaves Vout unchanged — only the current and power change.
Dropping voltage with resistors and Arduino analog read
The most common use of a resistor voltage divider is to safely measure a voltage that is higher than what a microcontroller can read. Arduino analog pins read at most 5 V (or 3.3 V on 3.3 V boards). To read a 12 V battery, you drop the voltage below the pin limit with a divider, then divide the analogRead value by the divider ratio to recover the real voltage. The same technique is used for level shifting — bringing a 5 V signal down to a 3.3 V input.
Tips: choosing resistor values and the load effect
- Don't use values that are too low: small resistors draw more current and waste energy as heat. Kilo-ohm (kΩ) values are usually a good balance between accuracy and wasted current.
- Mind the load effect: any load on the output sits in parallel with R2 and lowers Vout. A divider only gives the correct value into a high-impedance input (like an analog pin); it is not a power supply.
- Round to standard values: the calculated R2 may be an odd number. Pick the nearest E12/E24 standard resistor, or approximate it with a series/parallel combination.
Frequently Asked Questions
What is the voltage divider formula?
For a two-resistor voltage divider the output voltage is Vout = Vin × R2 / (R1 + R2). R2 is the resistor across which the output is taken (the one tied to ground). The output voltage is always less than or equal to the input voltage.
How do I find R2 for a target Vout?
If you know R1 and the desired output voltage, the required resistor is R2 = R1 × Vout / (Vin − Vout). For this to be valid the target Vout must be smaller than Vin, otherwise the result is negative or undefined.
Can I draw current from a voltage divider?
A voltage divider is meant to produce a reference or sense voltage, not to power a load. Any load connected to the output sits in parallel with R2 and pulls the output voltage down. That is why a divider is only reliable when feeding a high-impedance input such as an Arduino analog pin.
How do I read a high voltage with an Arduino?
Arduino analog pins can read at most 5 V (or 3.3 V). To measure a higher voltage such as a 12 V battery, use a voltage divider to scale it below the pin limit, then multiply the measured analogRead value by the divider ratio to recover the real voltage.