How this calculator works
The LM317 holds a reference voltage between OUT and ADJ. Divider current through R1 and the adjustment-pin current flow through R2 to ground, raising the output. This divider calculation does not verify dropout, load current, resistor power or heatsinking. Use the reference and adjustment current appropriate to the exact part and operating conditions.
Formula
Vout = Vref(1 + R2/R1) + Iadj R2; R2 = (Vout - Vref)/(Vref/R1 + Iadj).
Worked example
Ignoring adjustment current, R1 = 240 ohms and R2 = 720 ohms produce 5 V. Including 50 microamperes raises the estimate by 0.036 V to 5.036 V.
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Frequently asked questions
Can I ignore adjustment-pin current?
Set it to zero for the ideal divider estimate; it matters more when R2 is large.
Which resistor is R1?
R1 connects OUT to ADJ. R2 connects ADJ to ground.
Will the regulator always produce the calculated voltage?
No. Supply headroom, minimum load, thermal limits and component tolerances must also meet the datasheet requirements.