LM317 Calculator
Calculate LM317 output voltage, R1/R2 feedback resistors, current regulator resistor, dropout margin and heat dissipation for adjustable linear regulator circuits.
🔧 Practical Rule: LM317 is simple but it can get hot quickly. Always check P = (Vin − Vout) × Iload with the thermal tab. For switching alternatives, compare with the Boost Converter Calculator, Buck-Boost Converter Calculator and Voltage Regulator Heat Sink Calculator.
🔌 LM317 Adjustable Regulator Feedback Network
R1 / Upper Resistor
R2 / Lower Resistor
Reference Voltage
Iadj Optional
Input Voltage Optional
Load Current Optional
Presets:
The classic LM317 equation uses R1 from output to adjust pin and R2 from adjust pin to ground. R1 = 240Ω is commonly used.
Target Output Voltage
R1 Selected
Reference Voltage
Iadj Optional
Preferred Series
Input Voltage Check
Targets:
This tab finds R2 for your target voltage and suggests the nearest common resistor value.
Target Constant Current
Reference Voltage
Load Voltage Optional
Input Voltage Optional
Sense Resistor Rating
Regulator Dropout Allowance
Current:
LM317 can work as a simple current regulator using I ≈ 1.25 / Rsense.
Input Voltage
Output Voltage
Load Current
Ambient Temperature
Max Junction Temperature
Thermal Resistance θJA
Cases:
θJA is package + board/heatsink thermal resistance. Lower θJA means better cooling.
📐 Formula Reference
LM317 Output Voltage
Vout = Vref × (1 + R2/R1) + Iadj × R2
Design R2
R2 ≈ R1 × (Vout/Vref − 1)
Current Regulator
Iout ≈ Vref / Rsense
Heat Dissipation
Pd ≈ (Vin − Vout) × Iload
📋 Quick Reference
Common R1 = 240Ω
3.3V≈390Ω
5V≈720Ω
9V≈1.5kΩ
12V≈2.1kΩ
Current Source
20mA62Ω
100mA12.5Ω
350mA3.6Ω
1A1.25Ω
Design Checks
Headroom~3V
Linear heatcan be high
Use heatsinkif hot
Add capsfor stability
📚 Engineering Notes
R1 and R2 set the voltageR1 goes from output to adjust pin. R2 goes from adjust pin to ground. A common value for R1 is 240Ω.
Heat is often the real limitA 12V to 5V regulator at 1A burns about 7W as heat. That is too much without serious heatsinking.
Input must be higher than outputLM317 is not a boost converter. It needs input voltage above the output voltage plus dropout/headroom.
For high current, consider switching convertersFor battery-powered or high-current designs, compare with Buck-Boost or Boost Converter tools.
What is an LM317 Calculator?
An LM317 calculator helps design an adjustable voltage regulator circuit. It calculates the output voltage from R1 and R2, finds the required resistor value for a target output voltage, estimates current regulator resistor values, and checks power dissipation.
How to calculate LM317 output voltage
The common formula is Vout = 1.25 × (1 + R2/R1) + Iadj × R2. For many quick designs, the adjust pin current is small and the simpler formula gives a close estimate.
Why does LM317 get hot?
LM317 is a linear regulator, so the extra voltage is converted into heat. The heat is approximately (Vin − Vout) × Iload. A high input voltage and high load current can require a heat sink.
❓ Frequently Asked Questions
With R1 = 240Ω and ignoring Iadj, R2 is about 720Ω for roughly 5V output. This calculator also shows the nearest standard resistor value.
Use the LM317 formula Vout = 1.25 × (1 + R2/R1). With R1 = 240Ω, R2 is about 2.06kΩ for 12V, so a nearby standard value around 2.0kΩ to 2.1kΩ is commonly used depending on accuracy.
A practical quick estimate is output voltage plus about 3V headroom. The exact dropout depends on current, temperature and device variant, so always check the datasheet for final design.
It depends on power dissipation. If (Vin − Vout) × load current is high, the regulator can overheat. Use the heat check tab to estimate junction temperature and thermal margin.
Yes. Put a sense resistor between output and adjust pin. The current is approximately 1.25V divided by the sense resistor value.
Common causes include too little input headroom, excessive load current, wrong resistor wiring, poor grounding, thermal shutdown, or missing input/output capacitors.