Documentation sections

Via Thermal Resistance Calculator

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

Slug via-thermal-resistance. Calculate thermal via resistance, array conductance, and current capacity for PCB thermal management. Design filled and plated via arrays. Free, instant results.

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 via-thermal-resistance
Name	Unit	Range	Default	Description
viaDiameternumber	mm	0.05 … 6	0.3	Via Diameter
platingThicknessnumber	μm	1 … 100	25	Copper plating thickness on via barrel walls
boardThicknessnumber	mm	at least 0.1	1.6	Board Thickness
numViasnumber	none	1 … 100	4	Number of Vias
viaFillnumber	none	0 … 1	0	Via FillOne of: 0 Unfilled (plated) · 1 Copper filled
tempRisenumber	°C	1 … 100	10	Allowed temperature rise of the via barrel above ambient. Used only for the IPC-2221 current capacity; the thermal resistance does not depend on it.

Outputs

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

Outputs of via-thermal-resistance
Field	Unit	Description
viaResistance	°C/W	Single Via Thermal Resistance
arrayResistanceheadline	°C/W	Array Thermal Resistance
thermalConductance	W/°C	Thermal Conductance
currentCapacity	mA	IPC-2221 external-conductor fit, I = 0.048·ΔT^0.44·A^0.725 with A in mil², on the plated barrel (the full bore when copper-filled)

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": "via-thermal-resistance",
    "inputs": {
      "viaDiameter": 0.3,
      "platingThickness": 25,
      "boardThickness": 1.6,
      "numVias": 4,
      "viaFill": 0,
      "tempRise": 10
    }
  }'

import rftools

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

result = client.calculate("via-thermal-resistance", {
    "viaDiameter": 0.3,
    "platingThickness": 25,
    "boardThickness": 1.6,
    "numVias": 4,
    "viaFill": 0,
    "tempRise": 10,
})
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": "via-thermal-resistance",
    "inputs": {
      "viaDiameter": 0.3,
      "platingThickness": 25,
      "boardThickness": 1.6,
      "numVias": 4,
      "viaFill": 0,
      "tempRise": 10
    }
  }),
});
console.log(response.status, await response.json());

package main

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

func main() {
	body := []byte(`{
	  "slug": "via-thermal-resistance",
	  "inputs": {
	    "viaDiameter": 0.3,
	    "platingThickness": 25,
	    "boardThickness": 1.6,
	    "numVias": 4,
	    "viaFill": 0,
	    "tempRise": 10
	  }
	}`)
	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": "via-thermal-resistance",
  "values": {
    "viaResistance": 192.41400439917464,
    "arrayResistance": 48.10350109979366,
    "thermalConductance": 0.020788507637426196,
    "currentCapacity": 1685.3446055708444
  },
  "warnings": [],
  "errors": [],
  "provenance": {
    "method": "calculator:via-thermal-resistance",
    "version": "api@3f2c1a9b04de",
    "formulaRef": "IPC-2221A Figure 6-4 (conductor current capacity, and its fit); Fourier conduction for θ",
    "assumptions": [],
    "validRange": {
      "status": "inside",
      "bounds": {
        "viaDiameter": {
          "min": 0.05,
          "max": 6,
          "unit": "mm"
        },
        "platingThickness": {
          "min": 1,
          "max": 100,
          "unit": "μm"
        },
        "boardThickness": {
          "min": 0.1,
          "max": null,
          "unit": "mm"
        },
        "numVias": {
          "min": 1,
          "max": 100,
          "unit": ""
        },
        "viaFill": {
          "min": 0,
          "max": 1,
          "unit": ""
        },
        "tempRise": {
          "min": 1,
          "max": 100,
          "unit": "°C"
        }
      },
      "outside": [],
      "model": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "viaDiameter": 0.3,
      "platingThickness": 25,
      "boardThickness": 1.6,
      "numVias": 4,
      "viaFill": 0,
      "tempRise": 10
    },
    "seed": null,
    "elapsedSeconds": 0.003
  }
}

{
  "detail": [
    {
      "type": "float_parsing",
      "loc": [
        "body",
        "inputs",
        "viaDiameter"
      ],
      "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 pageVia Thermal Resistance Calculator, at these values