Voltage Divider Calculator — Output Voltage & Resistors

How the Voltage Divider Calculator works

The voltage divider calculator computes Vout for any combination of R1, R2, and Vin using the standard formula Vout = Vin × R2/(R1+R2). It also solves in reverse — enter the target output voltage and one resistor value to find the other. Embedded and electronics engineers use voltage dividers constantly for ADC input scaling, sensor signal conditioning, and reference voltage generation.

The voltage divider formula

A resistive voltage divider consists of two resistors in series connected between a supply voltage and ground. The junction between R1 and R2 produces Vout = Vin × R2/(R1+R2). For example, to scale a 12 V signal to a 3.3 V ADC input, R1 = 27 kΩ and R2 = 10 kΩ gives Vout = 12 × 10/(27+10) = 3.24 V — close enough for most ADC applications with ±1% resistors.

ADC input scaling and sensor interfacing

Microcontrollers with 3.3 V ADCs cannot directly read 5 V or 12 V sensors. A carefully chosen voltage divider scales the signal down safely. The calculator lets you enter the ADC reference voltage as the target Vout and the sensor supply as Vin, then select from E24 standard resistor values to find the closest achievable ratio — minimising the voltage scaling error without needing non-standard components.

Load effect and impedance matching

A voltage divider only behaves as calculated when the load impedance is much larger than R2. If the load resistance (Rload) is comparable to R2, the effective lower resistance becomes R2||Rload, pulling Vout below the ideal value. The rule of thumb is that Rload should be at least 10× R2. The tool displays the loaded output voltage when you enter a load resistance, showing the actual versus ideal split.

Power dissipation and resistor sizing

Power dissipated in the divider is P = Vin² / (R1+R2). For a 12 V divider with 1 kΩ total resistance, P = 144 mW — requiring at least a 1/4 W resistor. High total resistance reduces quiescent current but makes the divider more susceptible to load effects. The calculator shows dissipated power in R1 and R2 separately so you can choose the correct resistor wattage rating for your operating conditions.

Frequently asked questions

What is a voltage divider?

A voltage divider is a simple circuit with two resistors (R1 and R2) connected in series between a supply voltage and ground. The output voltage is taken at the junction between them. The formula is Vout = Vin × R2 / (R1 + R2). It produces a fraction of the input voltage determined by the ratio of the two resistors.

When should I use a voltage divider?

Voltage dividers are useful for scaling down voltages for level shifting — such as converting a 5V signal to 3.3V for a microcontroller input — for biasing transistor bases, and for creating stable reference voltages in sensor circuits. They are not suitable for powering loads that draw significant current, as the load current changes the output voltage unpredictably.

What is the loading effect?

When a load resistance is connected to the output of a voltage divider, it appears in parallel with R2. This reduces the effective resistance of the lower half of the divider, pulling Vout lower than the calculated value. The effect is significant when the load resistance is comparable to R2. To avoid loading effects, use a unity-gain op-amp buffer (voltage follower) between the divider output and the load.