Voltage Divider Calculator

Start by setting the desired output ratio: R2 / (R1 + R2) = Vout / Vin. This gives one equation in two unknowns, so you have freedom to choose the scale. The guiding constraints are: the divider current should be small enough not to drain the power source (so R1 + R2 should be large), but large enough that the load impedance Rload is at least 10 times R2 to minimize loading error. For general purpose bias and level-shifting circuits, the 1-100 kΩ range for R2 balances these requirements well. For battery-powered sensors with light loads, 100 kΩ to 1 MΩ is more appropriate. Always verify the power dissipation in each resistor stays within its rated wattage.

Yes, a resistive voltage divider works for AC as well as DC. The ratio Vout/Vin = R2 / (R1 + R2) applies to AC peak, RMS, or any other amplitude measure since both voltages scale together. However, for AC the situation is complicated by capacitive and inductive parasitics that change the effective impedance at higher frequencies. For power-frequency AC (50-60 Hz) measurement, a simple resistive divider works well as long as the resistors can handle the voltage and power. For galvanic isolation, use a transformer. For signals where the divider ratio must remain accurate across a wide bandwidth, use a compensated divider with matched RC time constants in each leg.

The clean Vout = Vin x R2 / (R1 + R2) equation only holds when nothing draws current from the tap. As soon as you connect a load resistance R_load across R2, it forms a parallel combination with R2 that is always smaller than R2 alone, pulling the output below the calculated value. The error grows as R_load approaches R2 and becomes negligible when R_load is much larger than R2. The rule of thumb is to keep R_load at least 10 times R2 to hold the error under about 10 percent, or buffer the divider with a unity-gain op-amp follower so the load draws no current from the tap.