LM317 Voltage Regulator Calculator
This calculator works out the output voltage of an LM317-style adjustable voltage regulator from the two resistors that set it. Enter R1, the resistor between the regulator's output and its adjust pin, and R2, the resistor between the adjust pin and ground, along with the reference voltage (Vref, typically 1.25 volts). It applies the standard formula, Vout equals Vref times one plus R2 over R1, and updates your output voltage instantly as you type. Alongside the headline voltage it also shows the resistor ratio (R2 divided by R1) and the reference voltage used, so you can see how each part of the sum contributes to the result. This is the calculation electronics hobbyists, students and technicians need when designing a power supply, choosing resistors to hit a target voltage, or checking an existing circuit against its datasheet. A common starting point is R1 = 240 ohms, which keeps the regulator's minimum load current satisfied; the worked example on this page uses R1 = 240 ohms and R2 = 720 ohms to give a 5.0 volt output. The small current from the adjust pin, typically around 50 microamps, is ignored here, though it can add a few millivolts of error when R2 is large. Treat the result as a design estimate based on ideal components; real-world resistor tolerances and regulator variation mean you should measure the actual output once built.
The formula
For an LM317-style adjustable regulator, Vout = Vref times (1 + R2 / R1), where Vref is about 1.25 volts, R1 sits between output and adjust, and R2 between adjust and ground. The small adjust pin current is ignored here.
Worked example
With R1 = 240 ohms and R2 = 720 ohms, Vout = 1.25 times (1 + 720/240) = 1.25 times 4 = 5.0 volts. Enter 240, 720, 1.25 to confirm.
Frequently asked questions
What is the LM317 output formula?
Vout = 1.25 times (1 + R2/R1). The 1.25 volt reference is scaled by the resistor ratio.
What value of R1 is recommended?
Commonly 240 ohms, which keeps the regulator’s minimum load current satisfied.
Does the adjust pin current matter?
It is small (about 50 microamps) and usually ignored, but it can add a few millivolts with large R2.
Who this calculator is for
This calculator is for electronics students, hobbyists and engineers.
What this calculator assumes
- You enter values in the units shown.
- Ideal components are assumed.
- Results are rounded for display.
Formula and sources
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