Voltage Divider Calculator

A voltage divider is two resistors in series across a voltage source, with the output taken from the junction between them. It's one of the most common building blocks in electronics — turning a fixed supply voltage into a smaller reference voltage, or scaling a sensor's output down to a range a microcontroller can read.

The formula and what it means

Vout = Vin × R2 ÷ (R1 + R2), where R2 is the resistor the output is measured across. Feed in 5V with R1 = 10kΩ and R2 = 10kΩ and the output is 2.5V — exactly half, because the resistors are equal and share the voltage drop equally. Change the ratio of R2 to the total and the output scales proportionally: a larger R2 relative to R1 gives a higher output voltage.

Solving for a resistor value instead

The same formula runs in reverse: given Vin, a target Vout and one resistor, the other resistor value can be worked out algebraically. This is the more common real task — you usually know the supply voltage and the voltage you need, and you're picking resistor values to hit it, often constrained to standard resistor series (E12, E24) rather than an arbitrary exact value.

Why loading the output matters

A voltage divider only holds its calculated output voltage if nothing meaningful is drawing current from that output node. Connect a load with a low resistance relative to R1 and R2, and it forms a second parallel path that pulls the output voltage down below the calculated value — this is called 'loading' the divider. The practical rule is to keep the divider's own current at least 10x the expected load current, or use a smaller pair of resistor values, so the output stays close to what the formula predicts.