Voltage Divider Calculator
Calculate output voltage (Vout), current, and power dissipation for any resistive voltage divider circuit. Switch to reverse mode to find the R1/R2 ratio for a desired output. Free and private — runs entirely in your browser.
How a Voltage Divider Works
A voltage divider is one of the most fundamental circuits in electronics. It uses two resistors connected in series across an input voltage source to produce a lower output voltage at the junction between them. The output voltage is determined by the ratio of the two resistors, making it predictable and easy to design. Voltage dividers are used in sensor circuits, biasing transistors, level shifting, ADC reference inputs, and potentiometer-based controls. This calculator handles all the math instantly, including current draw and power dissipation across each resistor.
Voltage Divider Formula
Vout = Vin x R2 / (R1 + R2)
Current: I = Vin / (R1 + R2)
Power: P1 = I^2 x R1 | P2 = I^2 x R2
Reverse: R1/R2 = (Vin / Vout) - 1
Voltage Divider for Microcontrollers
Microcontrollers like Arduino and ESP32 have ADC inputs rated for 3.3V or 5V. If you need to measure a higher voltage (such as a 12V battery), a voltage divider steps it down to a safe range. For example, to step 12V down to 3V, you need R1/R2 = 3 (e.g., R1 = 30k and R2 = 10k). Choose high-value resistors to minimize current draw and reduce power waste. This calculator shows current and power dissipation so you can verify your design stays within safe limits for your components.
Choosing Resistor Values
When designing a voltage divider, there are tradeoffs between accuracy and power consumption. Low-value resistors (100 ohm range) give better noise immunity but draw more current and waste power. High-value resistors (megohm range) save power but are more susceptible to noise and stray capacitance effects. A typical range is 1k to 100k for most applications. Use the reverse mode to find the ideal R1/R2 ratio for your desired output, then pick standard resistor values from the E24 or E96 series.
Loaded vs Unloaded Voltage Dividers
The voltage divider formula assumes no load at the output. In practice, if you connect a load with impedance comparable to R2, the effective resistance changes and Vout drops. For accurate output, the load impedance should be at least 10 times greater than R2. Op-amp buffers are commonly used to present a high-impedance input to the divider output while driving lower-impedance loads. Keep this in mind when designing precision voltage references.
Frequently Asked Questions
What is a voltage divider?
A voltage divider is a simple circuit with two resistors in series that produces an output voltage that is a fraction of the input voltage. The output depends on the ratio of the two resistors: Vout = Vin x R2 / (R1 + R2).
What is the voltage divider formula?
The formula is Vout = Vin x R2 / (R1 + R2), where Vin is the input voltage, R1 is the top resistor connected to Vin, and R2 is the bottom resistor connected to ground. The output is taken at the junction of R1 and R2.
How do I find R1 and R2 for a desired output voltage?
Use the reverse mode in this calculator. Enter Vin and your desired Vout. The calculator gives you the required R1/R2 ratio and suggests standard resistor pairs that achieve your target voltage with minimal error.
Does a voltage divider work with AC signals?
Yes, a resistive voltage divider works with both DC and AC signals. For AC, the same formula applies as long as both resistors are purely resistive. For reactive components (capacitors or inductors), you need impedance-based calculations.
Why does my voltage divider output drop when I connect a load?
Loading effect occurs because the load resistance appears in parallel with R2, reducing the effective resistance and lowering Vout. To minimize this, ensure the load impedance is at least 10x greater than R2, or use a buffer amplifier.
Is my data private?
Yes. All calculations run entirely in your browser using JavaScript. No data is sent to any server. Your circuit values never leave your device.