How to Use LM317 Calculator
- Use Calculate Vout when you already know R1 and R2. The common starting value for R1 is 240 Ω.
- Use Calculate R2 when you know the desired output voltage and want a resistor target.
- Choose Ω, kΩ, or MΩ before entering resistor values; the calculator converts the selected unit before applying the formula.
- Leave Iadj disabled for the usual simplified design estimate, or enable it and enter the adjustment current in microamps when you want a closer data-sheet calculation.
Formula & Theory - LM317 Calculator
Vout = Vref × (1 + R2 / R1) + Iadj × R2
R2 = (Vout - Vref) / (Vref / R1 + Iadj)
Common approximation: Vout ≈ 1.25 × (1 + R2 / R1)LM317 regulates approximately 1.25 V between the output and adjust pins. R1 sets the reference current, and R2 raises the output voltage above the reference.
The Iadj term is normally small, but it grows with R2. For high-value divider resistors or precision work, including Iadj can shift the final output enough to matter.
The nearest E24 hint is not a guarantee of exact output voltage. Standard resistor tolerances, regulator tolerance, load current, and temperature all affect the real circuit.
Use Cases for LM317 Calculator
- Choosing R1 and R2 for a bench supply or small analog rail.
- Checking how much output changes when a standard resistor value is substituted.
- Comparing the simplified equation with the data-sheet equation that includes Iadj.
- Documenting the resistor-divider calculation in a circuit note.