Documentation sections

LM317 Resistor Calculator

POSThttps://rftools.io/api/py/v1/calculate

Slug lm317-resistors. Calculate R2 resistor value for LM317/LM338 adjustable voltage regulator output voltage, with actual Vout and resistor power dissipation.

Inputs

Send every one of these in inputs, in the units shown. Most are required: a call without one is refused (400) and costs nothing; one the calculator can do without takes the default shown. An input outside its range is still computed and flagged in the provenance.

Inputs of lm317-resistors
Name	Unit	Range	Default	Description
voutnumber	V	1.25 … 37	5	Desired output voltage (minimum 1.25V = Vref)
r1number	Ω	1 … 10000	240	R1 from output pin to ADJ pin — typically 240Ω per LM317 datasheet
iadjnumber	μA	0 … 500	50	ADJ pin current — typically 50μA for LM317, 65μA for LM317A
eSeriesnumber	none	0 … 96	0	Rounds every part to the nearest value in the series; the response and the export are the rounded design's.One of: 0 Ideal values · 12 E12 (10%) · 24 E24 (5%) · 48 E48 (2%) · 96 E96 (1%)

Outputs

Returned in values, by key. A value that is unbounded or undefined is null.

Outputs of lm317-resistors
Field	Unit	Description
r2headline	Ω	R2 from the ADJ pin to ground: the exact value for R1, or with a part series the nearest value of the series
voutActual	V	The output voltage R1 and R2 set, including the ADJ pin current; with a part series, the rounded resistors' voltage
powerR1	mW	Power dissipated in R1
powerR2	mW	Power dissipated in R2

Calling it

Calculate: one call, the request and response in full, and its errors.
Batch: this calculator with others, or a sweep, in one request.
Solve for a target: the value of one input that brings an output to a target.

curl -X POST https://rftools.io/api/py/v1/calculate \
  -H "X-API-Key: $RFTOOLS_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "slug": "lm317-resistors",
    "inputs": {
      "vout": 5,
      "r1": 240,
      "iadj": 50,
      "eSeries": 0
    }
  }'

import rftools

client = rftools.Client()  # reads RFTOOLS_API_KEY, or the key `rftools login` saved

result = client.calculate("lm317-resistors", {"vout": 5, "r1": 240, "iadj": 50, "eSeries": 0})
print(result.values)
print(result.warnings, result.provenance.formula_ref)

const response = await fetch("https://rftools.io/api/py/v1/calculate", {
  method: "POST",
  headers: {
    "X-API-Key": process.env.RFTOOLS_API_KEY,
    "Content-Type": "application/json",
  },
  body: JSON.stringify({
    "slug": "lm317-resistors",
    "inputs": {
      "vout": 5,
      "r1": 240,
      "iadj": 50,
      "eSeries": 0
    }
  }),
});
console.log(response.status, await response.json());

package main

import (
	"bytes"
	"fmt"
	"io"
	"net/http"
	"os"
)

func main() {
	body := []byte(`{
	  "slug": "lm317-resistors",
	  "inputs": {
	    "vout": 5,
	    "r1": 240,
	    "iadj": 50,
	    "eSeries": 0
	  }
	}`)
	req, err := http.NewRequest("POST", "https://rftools.io/api/py/v1/calculate", bytes.NewReader(body))
	if err != nil {
		panic(err)
	}
	req.Header.Set("X-API-Key", os.Getenv("RFTOOLS_API_KEY"))
	req.Header.Set("Content-Type", "application/json")
	res, err := http.DefaultClient.Do(req)
	if err != nil {
		panic(err)
	}
	defer res.Body.Close()
	out, _ := io.ReadAll(res.Body)
	fmt.Println(res.Status, string(out))
}

application/json

{
  "slug": "lm317-resistors",
  "values": {
    "r2": 713.1537242472267,
    "voutActual": 5,
    "powerR1": 6.510416666666666,
    "powerR2": 19.718749999999996
  },
  "warnings": [],
  "errors": [],
  "provenance": {
    "method": "calculator:lm317-resistors",
    "version": "api@3f2c1a9b04de",
    "formulaRef": "Texas Instruments LM317 Datasheet (SNVS774)",
    "assumptions": [],
    "validRange": {
      "status": "inside",
      "bounds": {
        "vout": {
          "min": 1.25,
          "max": 37,
          "unit": "V"
        },
        "r1": {
          "min": 1,
          "max": 10000,
          "unit": "Ω"
        },
        "iadj": {
          "min": 0,
          "max": 500,
          "unit": "μA"
        },
        "eSeries": {
          "min": 0,
          "max": 96,
          "unit": ""
        }
      },
      "outside": [],
      "model": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "vout": 5,
      "r1": 240,
      "iadj": 50,
      "eSeries": 0
    },
    "seed": null,
    "elapsedSeconds": 0.003
  }
}

{
  "detail": [
    {
      "type": "float_parsing",
      "loc": [
        "body",
        "inputs",
        "vout"
      ],
      "msg": "Input should be a valid number, unable to parse string as a number",
      "input": "abc"
    }
  ]
}

An input sent as text. Nothing was counted.

{
  "detail": "More than 120 requests in 60 seconds from this address.",
  "errorKind": "rate_limited",
  "limit": 120,
  "windowSeconds": 60,
  "upgradeUrl": "https://rftools.io/pricing/"
}

Too many requests in a minute, with Retry-After.

Same inputs on the calculator pageLM317 Resistor Calculator, at these values