# Voltage Divider Calculator Calculate divider output voltage, solve for R1 or R2, and evaluate load effects with a circuit diagram > Canonical page: https://elysiatools.com/en/tools/voltage-divider-calculator - **Category:** Math & Numbers - **Keywords:** voltage divider, resistor divider, vout, potential divider, circuit, electronics, voltage, resistor, load regulation ## Overview A voltage divider taps a fraction of the input voltage across two series resistors. Equations: - Output voltage: Vout = Vin · R2 / (R1 + R2) - Solve R2: R2 = R1 / (Vin/Vout − 1) - Solve R1: R1 = R2 · (Vin/Vout − 1) Load effect: a load RL in parallel with R2 sags Vout; the tool reports both ideal and loaded output. ## Inputs - **Calculation Mode** (select) - **Supply Voltage (Vin)** (number): e.g. 12 - **Top Resistor (R1)** (number): e.g. 10000 (Ω) - **Bottom Resistor (R2)** (number): e.g. 10000 (Ω) - **Desired Output Voltage (Vout)** (number): required in find-R1/R2 mode, e.g. 3.3 - **Load Resistor (RL, optional)** (number): e.g. 100000 (Ω) — blank = no load ## When to use - When designing sensor interface circuits that require scaling down a high-voltage signal to match a microcontroller's ADC input range. - When selecting resistor values to establish a specific reference voltage in an analog circuit. - When analyzing the voltage sag caused by connecting a load resistor in parallel with the bottom resistor of a divider. ## How it works - Select the calculation mode: solve for output voltage (Vout), find the top resistor (R1), or find the bottom resistor (R2). - Input the supply voltage (Vin) along with the known resistor values or target output voltage. - Optionally enter a load resistance (RL) to evaluate how the load sags the output voltage. - Review the calculated values, current draw, and the ideal versus loaded output voltage. ## Use cases - Scaling a 5V sensor signal down to 3.3V for safe compatibility with a 3.3V microcontroller GPIO pin. - Calculating the exact resistor values needed to create a reference voltage for a comparator circuit. - Simulating the load regulation of a resistive divider when connected to a low-impedance analog input. ## Frequently asked questions ### What is a voltage divider? A simple passive circuit that scales down an input voltage using two series resistors. ### How does a load resistor (RL) affect the output voltage? A load resistor in parallel with R2 reduces the effective resistance of the bottom leg, causing the output voltage to sag. ### Can I use a voltage divider to power high-current devices? No, voltage dividers are inefficient for power delivery and suffer from severe voltage sag under heavy loads; use a voltage regulator instead. ### What formula is used to calculate the output voltage? The ideal output voltage is calculated using the formula Vout = Vin * R2 / (R1 + R2). ### How do I find the value of R2 for a target output voltage? 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