This free voltage divider calculator finds the output voltage (Vout) of a two-resistor divider using Vout = Vin × R2 / (R1 + R2). It also reports the series current through the divider, the power dissipated in R1 and R2, and the voltage-division ratio. It's a quick way to size resistors for reference voltages, sensor scaling, and bias networks — all in your browser.
A voltage divider is two resistors in series across a source voltage Vin. The output is taken across the lower resistor R2: Vout = Vin × R2 / (R1 + R2).
The same current flows through both resistors: I = Vin / (R1 + R2). Power in each resistor is P = I² × R. The division ratio R2 / (R1 + R2) is independent of Vin, so a divider scales any input by a fixed fraction.
Enter the input voltage Vin and the two resistor values R1 (top) and R2 (bottom). The calculator returns Vout, the series current, the power in each resistor, and the ratio. Keep units consistent (volts and ohms). To hit a target Vout, adjust the R1:R2 ratio — only the ratio matters for the output voltage, while the absolute values set the current and power.
An ideal divider assumes no current is drawn from Vout. In practice, any load connected to the output appears in parallel with R2 and pulls Vout down. As a rule of thumb, make the divider current at least 10× the load current (i.e. choose R1 and R2 low enough) so the load does not significantly change the output. Very high resistor values save power but make the output sensitive to loading and noise.
Voltage dividers create a smaller voltage from a larger one using two series resistors. Common uses include setting reference voltages, biasing transistors, scaling a high voltage down into the input range of an ADC or microcontroller, and reading resistive sensors (thermistors, photoresistors) as part of a divider.
The formula assumes no load on the output. When you connect a load, it sits in parallel with R2 and lowers the effective lower-leg resistance, reducing Vout. To minimize this, size the divider so its current is roughly 10× the load current, or buffer the output with an op-amp follower.
First pick the ratio R2 / (R1 + R2) to set Vout. Then scale both resistors to balance power and loading: smaller values stiffen the output (less affected by load) but waste more power; larger values save power but are noise- and load-sensitive. Values in the 1 kΩ–100 kΩ range are typical for low-power signal dividers.
The series current is I = Vin / (R1 + R2), and it flows continuously whenever the divider is powered. That current dissipates power P = Vin × I in the resistors even with no load. For battery-powered designs, use higher resistances (or switch the divider) to limit this standing current.