Calculate the output voltage of an LM317 adjustable regulator from R1 and R2, find the best standard resistor pair for a target voltage, or size the resistor for a constant-current (current-limiter) circuit. Add the input voltage and load current to check headroom, power dissipation and whether you need a heatsink.
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Regulator constants
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How to use the LM317 Voltage Regulator Calculator
Pick a mode: output voltage from resistors, resistors for a target voltage, or current limiter.
Enter your values: R1 and R2 in ohms, the target voltage, or the target current in mA.
Choose a resistor series (E12, E24 or 1% E96) for the standard-value search.
Add input voltage and load current to check headroom and heat; set the ambient temperature if it is hot.
Press Calculate and use the ranked table to pick parts you have on hand.
How the LM317 sets its output
The LM317 keeps a constant reference of about 1.25 V between its OUT and ADJ pins. R1 turns that voltage into a fixed current (1.25 V / R1); the same current flows through R2 to ground, so the voltage across R2 adds on top of the reference. That gives the familiar equation:
Vout = 1.25 V × (1 + R2 / R1) + Iadj × R2
Put a 0.1 µF ceramic capacitor on the input near the chip and about 1 µF on the output. A 10 µF capacitor from ADJ to ground improves ripple rejection; if you add it, protection diodes across the regulator are recommended for high output voltages.
Common LM317 resistor values (R1 = 240 Ω)
Target
Exact R2
E24 R2
Actual Vout*
3.3 V
390 Ω
390 Ω
3.30 V
5 V
713 Ω
680 Ω
4.83 V
6 V
903 Ω
910 Ω
6.03 V
9 V
1.47 kΩ
1.5 kΩ
9.14 V
12 V
2.04 kΩ
2 kΩ
11.77 V
13.8 V
2.38 kΩ
2.4 kΩ
13.87 V
15 V
2.61 kΩ
2.7 kΩ
15.45 V
*With Vref = 1.25 V and Iadj = 50 µA. Use the calculator's resistor mode to search other R1 values: for 5 V, R1 = 120 Ω with R2 = 360 Ω gives exactly 5.02 V.
Design tips
Minimum load: the LM317 needs a few milliamps flowing out to stay in regulation. R1 provides that path, so keep R1 at 240 Ω or lower (120 Ω for the 10 mA worst case).
Heat is the real limit: a linear regulator burns the whole voltage difference as heat. Dropping 12 V to 5 V at 1 A wastes 7 W, which needs a sizeable heatsink; a buck converter is usually better for large drops.
Constant current: in limiter mode the 1.25 V drop on the sense resistor plus the regulator's dropout means the input must be several volts above the load voltage.
Bigger relatives: the LM338 (5 A) and LM350 (3 A) use the same 1.25 V reference and the same equations, so this calculator works for them too.
Frequently asked questions
What is the LM317 output voltage formula?
Vout = Vref × (1 + R2 / R1) + Iadj × R2, where Vref is about 1.25 V between the OUT and ADJ pins, R1 connects OUT to ADJ, R2 connects ADJ to ground and Iadj is the adjust-pin current (typically 50 µA). The Iadj term is small, so Vout ≈ 1.25 × (1 + R2 / R1) is a good approximation.
What value should R1 be for an LM317?
240 Ω is the classic datasheet value, which passes about 5.2 mA. Because the worst-case minimum load current can be up to 10 mA, many designs use R1 = 120 Ω (about 10 mA) so the output stays regulated with no load attached. Values from 100 to 330 Ω are common.
How do I use an LM317 as a current limiter or LED driver?
Connect a single resistor R between OUT and ADJ and take the output from the ADJ side of the resistor. The regulator keeps 1.25 V across R, so I = 1.25 / R. For 350 mA, R = 1.25 / 0.35 ≈ 3.57 Ω, and the resistor dissipates 1.25 × 0.35 ≈ 0.44 W.
How much input voltage does an LM317 need?
The input must be higher than the output by at least the dropout voltage, which is roughly 1.5 to 2.5 V depending on load current and temperature. Allowing about 3 V of headroom is a safe rule. The input-to-output difference must also stay below 40 V for the standard part.
Does my LM317 need a heatsink?
Power dissipated in the regulator is (Vin − Vout) × load current. A TO-220 package in free air has a junction-to-ambient thermal resistance of roughly 50 °C/W, so it can handle only about 1.5–2 W at room temperature before the junction approaches 125 °C. Above that you need a heatsink; the calculator estimates the required heatsink rating in °C/W.
What is the output range and maximum current of the LM317?
The standard LM317 adjusts from 1.25 V to 37 V and can supply more than 1.5 A in the TO-220 package with adequate heatsinking. It has internal current limiting and thermal shutdown. Check your exact part's datasheet, since packages and variants differ.
Why is my LM317 output a little different from the calculation?
Resistor tolerance (5% for E24, 1% for E96), the Vref tolerance of the chip (about 1.2 to 1.3 V) and load effects all shift the output. For a precise voltage, use 1% resistors or replace part of R2 with a trimmer potentiometer.
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