Voltage Divider Calculator

A voltage divider uses two resistors in series to produce a lower voltage from a higher one. The output is Vout = Vin x R2 / (R1 + R2). With 12 V in, R1 = 10 kilohms and R2 = 20 kilohms, the output is 12 x 20,000 / 30,000 = 8 V.

R1 sits between the input and the output point, and R2 sits between the output and ground. The output is the share of the input that falls across R2. Equal resistors give half the input. Making R2 larger raises the output. Enter the input voltage and both resistors in ohms (type 10000 for 10 k), and the results update as you type. You also get the divider ratio, the current through the chain and the power used by each resistor.

Common uses are scaling a sensor or battery voltage down for a microcontroller's input, setting a reference voltage and biasing a transistor. One classic pair is 1 kilohm over 2 kilohms, which turns a 5 V signal into about 3.3 V for a 3.3 V chip.

The math assumes nothing else draws current from the output. A load connected there acts in parallel with R2 and pulls the voltage down. To allow for it, work out R2 x RL / (R2 + RL) and enter that as R2. A good rule is to keep the divider current at least ten times the load current.

A divider is also a poor way to power anything. It wastes energy and the output sags under load, so use a voltage regulator instead. The Resistor Color Code Calculator helps you find the parts.