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LM317 Voltage Calculator

Calculate the output voltage of an LM317 adjustable regulator from R1 and R2 (or the R2 needed for a target voltage).

Calculate

Output voltage

5 V

Vref = 1.25 V, Iadj (~50 µA) ignored. R1 is typically 240 Ω.

Disclaimer: This calculator is provided for general informational and educational purposes only, on an “as is” basis and without any warranty of accuracy or fitness for a particular purpose. Results may contain errors — always verify independently before relying on them in real designs. PartAndStock accepts no liability for any loss or damage arising from use of this tool, including when embedded on third-party sites.

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Step-by-step: How to Set the LM317 Voltage

How to use

1Choose the mode: find output voltage or find R2.
2For voltage, enter R1 (typically 240 Ω) and R2.
3The output voltage is calculated instantly.
4In find-R2 mode, enter target voltage + R1 to get the required R2 and nearest standard value.

How it works

Find the output voltage of the LM317 adjustable voltage regulator from the R1/R2 resistor values, or calculate the R2 needed for a target voltage.

Vout = 1.25 · (1 + R2/R1)  ·  R2 = R1 · (Vout/1.25 − 1)  ·  Vref = 1.25 V (Iadj ignored)

How the LM317 works

The LM317 holds a fixed 1.25 V reference between its ADJ (adjust) and OUT (output) pins. This voltage drives a fixed reference current through R1; the same current flows through R2, raising the output voltage in proportion to R2. The result: Vout = 1.25·(1 + R2/R1). R1 is usually kept fixed at 240 Ω and R2 is varied to set the output.

Choosing R1, minimum load and limits

R1 is typically 240 Ω (220 Ω in some designs); it provides the minimum load current (~10 mA) the LM317 needs to stay stable. The small Iadj current (~50 µA) out of the ADJ pin is ignored in most calculations. The output must be at least ~2–3 V of dropout below the input; at high current/voltage difference the LM317 heats up and needs a heatsink.

Worked examples

R1=240 Ω, R2=720 Ω → Vout = 1.25·(1+720/240) = 5.0 V
Target 12 V, R1=240 Ω → R2 = 240·(12/1.25−1) ≈ 2.06 kΩ (standard 2 kΩ)
Target 3.3 V, R1=240 Ω → R2 ≈ 394 Ω (standard 390 Ω)

Common Output Voltages (R1 = 240 Ω)

Output (Vout)	R2
1.25 V	0 Ω
3.3 V	390 Ω
5 V	720 Ω
9 V	1.5 kΩ
12 V	2.06 kΩ
15 V	2.64 kΩ

Frequently Asked Questions

What is the LM317 output voltage formula?+

Vout = 1.25 · (1 + R2/R1). 1.25 V is the LM317's ADJ–OUT reference voltage. E.g. R1=240 Ω, R2=720 Ω → 5 V.

What value should R1 be?+

Typically 240 Ω (220 Ω in some designs). This value provides the minimum load current the regulator needs to stay stable.

What is the maximum output of an LM317?+

The output can be up to ~2–3 V of dropout below the input. For example, from a 24 V input you can practically get up to ~21 V out.

Should the Iadj current be included?+

The ~50 µA current out of the ADJ pin is ignored in most calculations; it only causes a small deviation at very high R2 values.

How much current can the LM317 supply?+

Typically up to ~1.5 A; at that current it needs a suitable heatsink. For more current, use an external pass transistor or a different regulator.

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