Voltage divider calculator

The formula

A voltage divider (or potential divider) is two resistors in series across a supply. The output, taken from the middle, is Vout = Vin × R2 ÷ (R1 + R2), where R1 is the top resistor and R2 the bottom one. Enter any three of the four values and the calculator solves for the last.

Worked example

To get 3.3 V from 12 V with R2 = 10 kΩ: R1 = R2 × (Vin − Vout) ÷ Vout = 10k × 8.7 ÷ 3.3 ≈ 26.4 kΩ - in practice 27 kΩ, giving about 3.24 V. Remember a divider only holds its voltage with almost no load: anything drawing real current from the middle pulls Vout down.

Regulator feedback resistors

Adjustable regulators and switch-mode power supplies set their output with a divider into a feedback (FB or ADJ) pin. The regulator holds that pin at its reference voltage - 1.25 V on an LM317, often 0.8 V or 0.6 V on buck converters - so put the reference in as Vout and the output voltage you want as Vin to find the resistors. When a supply's output is wrong, a drifted feedback resistor is a common cause - check it against this calculation. Power-supply boards with their parts marked are in our board library.

Questions

Which resistor is R1?

R1 is the top resistor, between the input voltage and the output; R2 is the bottom one, between the output and ground.

Why is my measured voltage lower than calculated?

Whatever is connected to the output - or your meter on high-value resistors - draws current and loads the divider. Keep the load current well below the current flowing through the resistors.