Documentation sections

LED Current Limiting Resistor Calculator

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

Slug led-resistor. Calculate the correct current limiting resistor for an LED. Shows exact value, nearest E24 standard, actual current, and 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 led-resistor
Name	Unit	Range	Default	Description
vSupplynumber	V	0.1 … 48	5	Supply Voltage
vForwardnumber	V	0.5 … 5	2	LED Forward Voltage
iForwardnumber	mA	0.1 … 1000	20	Desired Current
eSeriesnumber	none	0 … 96	24	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 led-resistor
Field	Unit	Description
rExactheadline	Ω	Exact Resistance
rNearest	Ω	The exact resistance rounded to the nearest value of the part series (E24 unless another is chosen); the exact value with ideal values
iActual	mA	The LED current through the series value
power	W	Power (exact R)
pNearest	W	The power the series value dissipates

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": "led-resistor",
    "inputs": {
      "vSupply": 5,
      "vForward": 2,
      "iForward": 20,
      "eSeries": 24
    }
  }'

import rftools

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

result = client.calculate("led-resistor", {"vSupply": 5, "vForward": 2, "iForward": 20, "eSeries": 24})
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": "led-resistor",
    "inputs": {
      "vSupply": 5,
      "vForward": 2,
      "iForward": 20,
      "eSeries": 24
    }
  }),
});
console.log(response.status, await response.json());

package main

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

func main() {
	body := []byte(`{
	  "slug": "led-resistor",
	  "inputs": {
	    "vSupply": 5,
	    "vForward": 2,
	    "iForward": 20,
	    "eSeries": 24
	  }
	}`)
	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": "led-resistor",
  "values": {
    "rExact": 150,
    "rNearest": 150,
    "iActual": 20,
    "power": 0.06,
    "pNearest": 0.06
  },
  "warnings": [],
  "errors": [],
  "provenance": {
    "method": "calculator:led-resistor",
    "version": "api@3f2c1a9b04de",
    "formulaRef": null,
    "assumptions": [],
    "validRange": {
      "status": "inside",
      "bounds": {
        "vSupply": {
          "min": 0.1,
          "max": 48,
          "unit": "V"
        },
        "vForward": {
          "min": 0.5,
          "max": 5,
          "unit": "V"
        },
        "iForward": {
          "min": 0.1,
          "max": 1000,
          "unit": "mA"
        },
        "eSeries": {
          "min": 0,
          "max": 96,
          "unit": ""
        }
      },
      "outside": [],
      "model": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "vSupply": 5,
      "vForward": 2,
      "iForward": 20,
      "eSeries": 24
    },
    "seed": null,
    "elapsedSeconds": 0.003
  }
}

{
  "detail": [
    {
      "type": "float_parsing",
      "loc": [
        "body",
        "inputs",
        "vSupply"
      ],
      "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 pageLED Current Limiting Resistor Calculator, at these values