Documentation sections

Junction Temperature Calculator

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

Slug junction-temperature. Calculate semiconductor junction temperature from power dissipation and thermal resistance chain (θJC + θCS + θSA). Essential for transistor, MOSFET, and IC thermal design.

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 junction-temperature
Name	Unit	Range	Default	Description
pdnumber	W	at least 0	1	Power dissipated in the device (Watts)
tanumber	°C	−55 … 125	25	Ambient (surrounding air) temperature
thetaJCnumber	°C/W	at least 0	5	Junction-to-case thermal resistance from device datasheet
thetaCSnumber	°C/W	at least 0	0.5	Case-to-heatsink thermal resistance (thermal pad/paste). ~0.1–1.0 °C/W typical.
thetaSAnumber	°C/W	at least 0	10	Heatsink-to-ambient thermal resistance. Lower = better heatsink. ~1–50 °C/W typical.
numLayersnumber	none	0 … 6	0	Thermal layers in series between the case and the heatsink besides θ_CS, such as an insulating pad, a spreader or a board with thermal vias (0–6)
layer1Thetanumber	°C/W	0 … 100	1	Thermal resistance of extra layer 1, in series between the case and the heatsink
layer2Thetanumber	°C/W	0 … 100	1	Thermal resistance of extra layer 2, in series between the case and the heatsink
layer3Thetanumber	°C/W	0 … 100	1	Thermal resistance of extra layer 3, in series between the case and the heatsink
layer4Thetanumber	°C/W	0 … 100	1	Thermal resistance of extra layer 4, in series between the case and the heatsink
layer5Thetanumber	°C/W	0 … 100	1	Thermal resistance of extra layer 5, in series between the case and the heatsink
layer6Thetanumber	°C/W	0 … 100	1	Thermal resistance of extra layer 6, in series between the case and the heatsink

Outputs

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

Outputs of junction-temperature
Field	Unit	Description
thetaJA	°C/W	Total junction-to-ambient thermal resistance: θJA = θJC + θCS + Σθ_L + θSA
tjheadline	°C	Device junction temperature: TJ = TA + PD × θJA
tc	°C	Device case/package temperature
ts	°C	Heatsink temperature
margin	°C	Temperature margin below 125°C maximum junction temperature. Positive = safe.

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": "junction-temperature",
    "inputs": {
      "pd": 1,
      "ta": 25,
      "thetaJC": 5,
      "thetaCS": 0.5,
      "thetaSA": 10,
      "numLayers": 0,
      "layer1Theta": 1,
      "layer2Theta": 1,
      "layer3Theta": 1,
      "layer4Theta": 1,
      "layer5Theta": 1,
      "layer6Theta": 1
    }
  }'

import rftools

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

result = client.calculate("junction-temperature", {
    "pd": 1,
    "ta": 25,
    "thetaJC": 5,
    "thetaCS": 0.5,
    "thetaSA": 10,
    "numLayers": 0,
    "layer1Theta": 1,
    "layer2Theta": 1,
    "layer3Theta": 1,
    "layer4Theta": 1,
    "layer5Theta": 1,
    "layer6Theta": 1,
})
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": "junction-temperature",
    "inputs": {
      "pd": 1,
      "ta": 25,
      "thetaJC": 5,
      "thetaCS": 0.5,
      "thetaSA": 10,
      "numLayers": 0,
      "layer1Theta": 1,
      "layer2Theta": 1,
      "layer3Theta": 1,
      "layer4Theta": 1,
      "layer5Theta": 1,
      "layer6Theta": 1
    }
  }),
});
console.log(response.status, await response.json());

package main

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

func main() {
	body := []byte(`{
	  "slug": "junction-temperature",
	  "inputs": {
	    "pd": 1,
	    "ta": 25,
	    "thetaJC": 5,
	    "thetaCS": 0.5,
	    "thetaSA": 10,
	    "numLayers": 0,
	    "layer1Theta": 1,
	    "layer2Theta": 1,
	    "layer3Theta": 1,
	    "layer4Theta": 1,
	    "layer5Theta": 1,
	    "layer6Theta": 1
	  }
	}`)
	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": "junction-temperature",
  "values": {
    "thetaJA": 15.5,
    "tj": 40.5,
    "tc": 35.5,
    "ts": 35,
    "margin": 84.5
  },
  "warnings": [],
  "errors": [],
  "provenance": {
    "method": "calculator:junction-temperature",
    "version": "api@3f2c1a9b04de",
    "formulaRef": null,
    "assumptions": [],
    "validRange": {
      "status": "inside",
      "bounds": {
        "pd": {
          "min": 0,
          "max": null,
          "unit": "W"
        },
        "ta": {
          "min": -55,
          "max": 125,
          "unit": "°C"
        },
        "thetaJC": {
          "min": 0,
          "max": null,
          "unit": "°C/W"
        },
        "thetaCS": {
          "min": 0,
          "max": null,
          "unit": "°C/W"
        },
        "thetaSA": {
          "min": 0,
          "max": null,
          "unit": "°C/W"
        },
        "numLayers": {
          "min": 0,
          "max": 6,
          "unit": ""
        },
        "layer1Theta": {
          "min": 0,
          "max": 100,
          "unit": "°C/W"
        },
        "layer2Theta": {
          "min": 0,
          "max": 100,
          "unit": "°C/W"
        },
        "layer3Theta": {
          "min": 0,
          "max": 100,
          "unit": "°C/W"
        },
        "layer4Theta": {
          "min": 0,
          "max": 100,
          "unit": "°C/W"
        },
        "layer5Theta": {
          "min": 0,
          "max": 100,
          "unit": "°C/W"
        },
        "layer6Theta": {
          "min": 0,
          "max": 100,
          "unit": "°C/W"
        }
      },
      "outside": [],
      "model": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "pd": 1,
      "ta": 25,
      "thetaJC": 5,
      "thetaCS": 0.5,
      "thetaSA": 10,
      "numLayers": 0,
      "layer1Theta": 1,
      "layer2Theta": 1,
      "layer3Theta": 1,
      "layer4Theta": 1,
      "layer5Theta": 1,
      "layer6Theta": 1
    },
    "seed": null,
    "elapsedSeconds": 0.003
  }
}

{
  "detail": [
    {
      "type": "float_parsing",
      "loc": [
        "body",
        "inputs",
        "pd"
      ],
      "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 pageJunction Temperature Calculator, at these values