LM317 Calculator
Enter your R1 and R2 resistor values to calculate the output voltage of an LM317 adjustable voltage regulator circuit.
What this calculator does
The LM317 is a widely used adjustable three-terminal linear voltage regulator. Its output voltage is set by two external resistors, R1 (between the output and adjustment pins) and R2 (between the adjustment pin and ground).
The formula
The standard datasheet formula is Vout = Vref × (1 + R2/R1) + Iadj × R2, where Vref is the LM317’s internal reference voltage (typically 1.25V) and Iadj is the small adjustment-pin current (typically about 50µA). The Iadj term is usually tiny compared to the main term, but this calculator includes it for accuracy.
This tool is for general electronics planning and education. Always check your specific LM317 datasheet, since Vref and Iadj can vary slightly between manufacturers and part variants, and add appropriate tolerances for real designs.