Voltage Divider Calculator

Find the output voltage of a two-resistor divider from the input voltage and resistor values using Vout = Vin x R2 / (R1 + R2).

This voltage divider calculator is for electronics students, makers, and engineers who need a smaller voltage from a larger one using two resistors. It is the tool you reach for when scaling a sensor signal down to fit a microcontroller's input range, setting a reference voltage, biasing a transistor, or reading a battery's level through an analog pin. Anyone learning how resistors share a voltage, or anyone who wants to skip rearranging the divider formula by hand, will find it quicker and less error-prone than working it out on paper.

A voltage divider puts two resistors in series across a source. The output is taken from the point between them. Because the same current flows through both resistors, each one takes a share of the total voltage in proportion to its resistance. The output across the bottom resistor R2 is Vout = Vin x R2 / (R1 + R2). The calculator adds R1 and R2, divides R2 by that total to get the ratio, and multiplies by your input voltage. It also reports the voltage dropped across R1, which is simply the input minus the output, so you can see how the source voltage splits between the two parts.

Say you have a 9 volt supply and you want roughly two-thirds of it. Put a 1,000 ohm resistor on top (R1) and a 2,000 ohm resistor on the bottom (R2). The total is 3,000 ohms, and R2 is two-thirds of that, so the ratio is 0.667. Multiply 9 volts by 0.667 and the output is 6 volts, with the remaining 3 volts dropped across R1. Swap to two equal 1,000 ohm resistors and the ratio becomes 0.5, so a 5 volt input gives 2.5 volts out - a clean halving.

Resistor physics is the same everywhere, so the math does not change by country. What differs is the labelling convention you may see on parts and schematics: resistor values follow the international E-series (E12, E24, and so on), and color-band codes are standard worldwide. Some regions favor showing values as 4k7 instead of 4.7k, but the number is identical. This calculator works in plain ohms and volts, so enter whatever your parts are marked once you convert any kilohm or megohm values.

• Loading the output without accounting for it. The simple formula assumes nothing draws current from Vout. If your load resistance is not much larger than R2, the real output sags below the calculated value.
• Choosing very large resistor values to save power, then finding the divider is too weak to drive the next stage, or that it picks up noise.
• Choosing very small resistor values, which waste power and can overload the source.
• Mixing up which resistor is R1 and which is R2. The output is always measured across the resistor connected to ground, which this calculator treats as R2.
• Forgetting to convert kilohms to ohms, which shifts the answer but, because it is a ratio, often hides the slip until a loaded test reveals it.

There is no deadline in electronics, but there is a design check worth making every time: confirm the current through the divider and the power each resistor handles. Power in a resistor is voltage across it times the current through it. A divider that runs hot, or one whose output collapses the moment you connect a load, points to resistor values that are too small or too large for the job. Prototype and measure before committing the values to a board.

• Voltage divider - Reference (retrieved 2026-06-11)