LM317 Voltage Regulator Calculator
Output of an LM317-type adjustable regulator from R1, R2 and the datasheet reference, or R2 for a target output, with dropout check and dissipation.
The resistor pair that sets the output you want and whether the input and the heatsink can support it — using the reference, adjust current and dropout from the part you are actually using.
Example: R1 = 240 Ω and R2 = 2.4 kΩ with V_ref 1.25 V and I_adj 50 µA give 13.87 V. For 5.00 V, R2 = 713 Ω; from 12 V at 0.5 A the regulator dissipates 3.56 W and needs a heatsink.
1.25 V across R1,
scaled up by R2.
The output equation, why the datasheet values are yours to enter, and the two checks that decide whether the circuit works.
The output equation
The regulator holds its reference voltage between the output and adjust pins, so R1 always carries V_ref ÷ R1. That current, plus the small adjust-pin current, flows through R2 and lifts the adjust pin above ground: V_out = V_ref (1 + R2 ÷ R1) + I_adj × R2. Solving for R2 at a target output is one rearrangement. The minimum output is V_ref itself (R2 = 0); the maximum is set by the input and the part's ratings.
Why V_ref, I_adj and dropout are inputs
The 1.25 V reference and 50 µA adjust current shown are labelled samples typical of the LM317 family; datasheets give each with a tolerance and temperature drift, and other adjustable regulators (LM1117, LT1085, low-dropout parts) use different references. Enter the figures for your part. Keeping R1 small enough that the divider current swamps I_adj is what makes the output insensitive to that current; the page flags when the I_adj term exceeds 1 % of the output.
Dropout and dissipation
A linear regulator needs its input above the output by at least the dropout voltage (from the datasheet, at your load current) or it stops regulating; the page compares. Everything between input and output at the load current becomes heat, (V_in − V_out) × I, plus the small divider current from the input; above about a watt a heatsink is usually needed, sized from the package thermal resistance and the maximum junction temperature in the datasheet, which this page does not carry. Nothing leaves the browser; the same four anonymous usage counts as the rest of the site apply.
SOURCES
- Texas Instruments, LM317 3-Terminal Adjustable Regulator datasheet (SLVS044) — output voltage equation, reference and adjust-current specifications (values entered by the user)
Last reviewed 19 September 2026. How results are checked: How we verify.