Voltage Divider Calculator

A voltage divider splits an input voltage across two resistors in series. The output voltage across R2 is Vout = Vin × R2 / (R1 + R2) — the fraction R2/(R1+R2) determines how much of the input voltage appears at the output. For example, with Vin = 9V, R1 = 1kΩ, and R2 = 2kΩ, Vout = 9 × 2000/3000 = 6V.

The voltage divider formula is Vout = Vin × R2 / (R1 + R2), where Vin is the input voltage, R1 is the resistor connected to the input side, and R2 is the resistor connected to ground. The output voltage is measured across R2. For example, with Vin = 9 V, R1 = 1 kΩ, and R2 = 2 kΩ, Vout = 9 × 2000 / (1000 + 2000) = 6 V.

Because R1 and R2 are in series, the same current flows through both resistors. That current is found with Ohm's law applied to the total resistance: I = Vin / (R1 + R2). Using the 9 V, 1 kΩ, 2 kΩ example, I = 9 / (1000 + 2000) = 0.003 A, or 3 mA.

The simple voltage divider formula assumes nothing else draws current from the R1–R2 junction. If you connect a load (like a sensor or another circuit) across R2, it forms a parallel combination with R2, lowering the effective resistance on that leg and pulling Vout down below the unloaded value. This is often called the 'loading effect,' and it's why voltage dividers are best used to feed high-impedance inputs (like an op-amp or ADC) rather than to directly power a component that draws significant current.

First decide the desired Vout/Vin ratio, which fixes the ratio R2 / (R1 + R2). Then pick an absolute resistance scale: smaller resistors (hundreds of ohms) draw more current and dissipate more power but resist noise and loading effects better; larger resistors (tens to hundreds of kΩ) save power but make the output more sensitive to whatever load is attached. A common compromise for signal-level dividers is to keep R1 and R2 in the 1 kΩ–100 kΩ range.

Yes, this is one of the most common uses — for example, dividing a 5 V sensor signal down to 3.3 V for a microcontroller's ADC pin. Choose R1 and R2 so that Vout = Vin × R2 / (R1 + R2) equals your target voltage at the expected input voltage, and keep the resistor values small enough (relative to the input impedance of the ADC) that loading error stays negligible.

Each resistor dissipates power according to P = I² × R, where I is the current flowing through the divider (I = Vin / (R1 + R2)). The total power drawn from the source is P = Vin × I = Vin² / (R1 + R2). This continuous power draw is the main downside of resistive voltage dividers compared to active regulators, especially when R1 + R2 is small.