# [Voltage Divider Calculator](https://blog.hirnschall.net/tools/voltage-divider/) author: [Sebastian Hirnschall](https://blog.hirnschall.net/about/) meta description: Calculate voltage divider output. Solve for V_out, V_in, R1, or R2 given any three. Divider ratio, current, power, and optional load effect included. meta title: Voltage Divider Calculator — Solve for V_out, V_in, R1, or R2 date published: 11.05.2026 (DD.MM.YYYY format) date last modified: 11.05.2026 (DD.MM.YYYY format) --- Calculator ---------- Provide any three of \( V\_{\text{in}} \), \( R\_1 \), \( R\_2 \), \( V\_{\text{out}} \) — the fourth is solved. The divider ratio, current, and power are always shown. \( R\_L \) is optional: if provided, the loaded output voltage is also calculated. * Input voltage (Vin): * V mV kV * Top resistor (R1): * Ω kΩ MΩ * Bottom resistor (R2): * Ω kΩ MΩ * Output voltage (Vout): * V mV kV --- * Load resistance (RL) — optional: * Ω kΩ MΩ Provide any three of V\_in, R1, R2, V\_out to calculate. Calculate Voltage Divider — Explanation ----------------------------- A voltage divider consists of two resistors connected in series between an input voltage and ground. The output is taken from the junction between them. Because the same current flows through both resistors, the voltage splits in proportion to the resistance values: \[ V\_{\text{out}} = V\_{\text{in}} \cdot \frac{R\_2}{R\_1 + R\_2} \] Where \( R\_1 \) is the top resistor (between \( V\_{\text{in}} \) and the output node) and \( R\_2 \) is the bottom resistor (between the output node and ground). The ratio \( R\_2 / (R\_1 + R\_2) \) is always between 0 and 1, so \( V\_{\text{out}} \) is always less than \( V\_{\text{in}} \). A voltage divider can only scale down, not up. The current through the divider and total power dissipated are: \[ I = \frac{V\_{\text{in}}}{R\_1 + R\_2}, \qquad P = V\_{\text{in}} \cdot I = \frac{V\_{\text{in}}^2}{R\_1 + R\_2} \] Higher total resistance reduces quiescent current and standby power, which matters in battery-powered designs. The tradeoff is that high-resistance dividers are more sensitive to loading, as described below. Loading Effect -------------- The formula above assumes no current is drawn from the output. In practice, any load connected to \( V\_{\text{out}} \) forms a parallel combination with \( R\_2 \), reducing the effective bottom resistance and pulling the output voltage down: \[ R\_{2,\text{eff}} = \frac{R\_2 \cdot R\_L}{R\_2 + R\_L}, \qquad V\_{\text{out,loaded}} = V\_{\text{in}} \cdot \frac{R\_{2,\text{eff}}}{R\_1 + R\_{2,\text{eff}}} \] As a rule of thumb, the loading error stays below about 10% when \( R\_L \geq 10 \cdot R\_2 \). For a stiff divider that changes little under load, keep the divider resistors well below the expected load impedance. The optional \( R\_L \) field in the calculator shows the exact loaded output and the percentage drop for any combination. Note that the Wheatstone bridge is essentially two voltage dividers sharing the same supply, with the measurement taken as the difference between their outputs. See the [Wheatstone bridge calculator](https://blog.hirnschall.net/tools/wheatstone-bridge/) for that case. Purpose of the Calculator ------------------------- Any three of \( V\_{\text{in}} \), \( R\_1 \), \( R\_2 \), \( V\_{\text{out}} \) determine the fourth. The rearranged formulas are: * To solve for \( V\_{\text{out}} \) (checking what a known divider produces): \[ V\_{\text{out}} = V\_{\text{in}} \cdot \frac{R\_2}{R\_1 + R\_2} \] * To solve for \( V\_{\text{in}} \) (finding what input produces a target output): \[ V\_{\text{in}} = V\_{\text{out}} \cdot \frac{R\_1 + R\_2}{R\_2} \] * To solve for \( R\_1 \) (selecting the top resistor given a known \( R\_2 \) and target ratio): \[ R\_1 = R\_2 \cdot \frac{V\_{\text{in}} - V\_{\text{out}}}{V\_{\text{out}}} \] * To solve for \( R\_2 \) (selecting the bottom resistor given a known \( R\_1 \) and target ratio): \[ R\_2 = R\_1 \cdot \frac{V\_{\text{out}}}{V\_{\text{in}} - V\_{\text{out}}} \] In all cases the divider ratio, quiescent current, and dissipated power are shown as additional outputs. Resistor Selection ------------------ Choosing absolute resistor values involves a tradeoff between loading stiffness and power consumption. Lower resistance values make the divider output less sensitive to load current variations but increase the standing current drawn from the supply. Higher values reduce quiescent power but make the output more susceptible to loading and to noise picked up on the output node. In most microcontroller ADC reference dividers, resistor values in the 10 kΩ to 100 kΩ range strike a reasonable balance. For voltage monitoring with high-impedance op-amp inputs, values up to 1 MΩ are workable. For power-supply feedback networks where a stable reference voltage is critical, lower values (1 kΩ to 10 kΩ) are more common to swamp out any leakage currents. Related Tools ------------- * [Wheatstone Bridge Calculator](https://blog.hirnschall.net/tools/wheatstone-bridge/) — two voltage dividers sharing a supply; measures unknown resistance from the differential output. * [RC Time Constant Calculator](https://blog.hirnschall.net/tools/rc-time-constant/) — adding a capacitor across R2 turns the divider into an RC low-pass filter. More calculators: [blog.hirnschall.net/tools/](https://blog.hirnschall.net/tools/).