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LM317 Voltage Calculator

A handy online output voltage calculator for the LM317, LM338, and other linear voltage regulators

The LM317 (LM317T), and high current LM338 (LM338T) are voltage regulators which can take an input voltage of 3-40 Volts DC, and output a fixed output voltage from 1.2 to 37 Volts DC.


The output voltage from the LM317 and LM338 is set using two resistors (R1 and R2) with their values chosen according to the following equation:

VOUT = 1.25 * ( 1 + R2/R1 )

LM317/LM338 Voltage Calculator

Below is our automated voltage calculator for the LM317 and LM338 regulators. Simply enter your target output voltage Vout and chosen value for R1 (100 to 1000 Ohms), click Calculate, and the required value of R2 will be displayed.

Vout: Target Output Voltage 1.5 to 37.0 Volts

R1: R1 Resistor Value 100-1000 Ohms

R2: Click Calculate to display the calculated R2 value here.

The results given above can be used to put together a voltage regulator, but the calculated value of R2 will not be a standard/stock resistor value. Since the accuracy of the voltage regulator itself will be up to 5% out, and resistor values are not exact, it is common to use a Potentiometer (aka variable resistor or trim pot) for some or all of R2. This enables the output voltage to be manually fine-tuned to provide the exact voltage required.

Enter values of R1 and R2 below to calculate the corresponding value of Vout.

R1: R1 Resistor Value 100-1000 Ohms

R2: R2 Resistor Value

Vout: Click Calculate to display the calculated Vout value here.