Documentation sections

PCB Controlled Impedance Calculator

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

Slug controlled-impedance. Calculate characteristic impedance for surface microstrip, embedded microstrip, and stripline PCB traces. Get Z0, effective Er, and target trace width. 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 controlled-impedance
Name	Unit	Range	Default	Description
traceTypenumber	none	0 … 2	0	Trace TypeOne of: 0 Surface microstrip · 1 Embedded microstrip · 2 Stripline
traceWidthnumber	mm	0.05 … 20	1.8	Trace Width
substrateHeightnumber	mm	0.01 … 10	0.2	Substrate Height
dielectricConstnumber	none	at least 1	4.4	Dielectric Constant
copperThicknessnumber	μm	at least 5	35	Copper Thickness
coverHeightnumber	mm	at least 0.01	0.1	Cover layer height (for embedded microstrip only)
coverDielectricnumber	none	at least 1	3.5	Permittivity of the covering layer (for embedded microstrip only)
frequencynumber	GHz	0.001 … 300	2.4	Used only for the guided wavelength; the impedance does not depend on it in this model

Outputs

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

Outputs of controlled-impedance
Field	Unit	Description
impedanceheadline	Ω	Characteristic Impedance
effectiveDielectric	none	Effective Dielectric Constant
propagationDelay	ps/mm	Propagation Delay
traceWidthFor50	mm	Trace Width for 50Ω
guidedWavelength	mm	Guided Wavelength
quarterWavelength	mm	Quarter Wavelength

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": "controlled-impedance",
    "inputs": {
      "traceType": 0,
      "traceWidth": 1.8,
      "substrateHeight": 0.2,
      "dielectricConst": 4.4,
      "copperThickness": 35,
      "coverHeight": 0.1,
      "coverDielectric": 3.5,
      "frequency": 2.4
    }
  }'

import rftools

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

result = client.calculate("controlled-impedance", {
    "traceType": 0,
    "traceWidth": 1.8,
    "substrateHeight": 0.2,
    "dielectricConst": 4.4,
    "copperThickness": 35,
    "coverHeight": 0.1,
    "coverDielectric": 3.5,
    "frequency": 2.4,
})
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": "controlled-impedance",
    "inputs": {
      "traceType": 0,
      "traceWidth": 1.8,
      "substrateHeight": 0.2,
      "dielectricConst": 4.4,
      "copperThickness": 35,
      "coverHeight": 0.1,
      "coverDielectric": 3.5,
      "frequency": 2.4
    }
  }),
});
console.log(response.status, await response.json());

package main

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

func main() {
	body := []byte(`{
	  "slug": "controlled-impedance",
	  "inputs": {
	    "traceType": 0,
	    "traceWidth": 1.8,
	    "substrateHeight": 0.2,
	    "dielectricConst": 4.4,
	    "copperThickness": 35,
	    "coverHeight": 0.1,
	    "coverDielectric": 3.5,
	    "frequency": 2.4
	  }
	}`)
	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": "controlled-impedance",
  "values": {
    "impedance": 15.856282795523272,
    "effectiveDielectric": 3.8376255770810888,
    "propagationDelay": 6.534473439728769,
    "traceWidthFor50": 0.34448241949142255,
    "guidedWavelength": 63.76438293151309,
    "quarterWavelength": 15.941095732878273
  },
  "warnings": [
    "Large deviation from 50Ω — adjust trace width"
  ],
  "errors": [],
  "provenance": {
    "method": "calculator:controlled-impedance",
    "version": "api@3f2c1a9b04de",
    "formulaRef": "Hammerstad & Jensen, IEEE MTT-S 1980; stripline core verified against the field solver committed in src/lib/pcb/__tests__/solver. The covered-microstrip filling factors are fitted to a layered solve.",
    "assumptions": [],
    "validRange": {
      "status": "inside",
      "bounds": {
        "traceType": {
          "min": 0,
          "max": 2,
          "unit": ""
        },
        "traceWidth": {
          "min": 0.05,
          "max": 20,
          "unit": "mm"
        },
        "substrateHeight": {
          "min": 0.01,
          "max": 10,
          "unit": "mm"
        },
        "dielectricConst": {
          "min": 1,
          "max": null,
          "unit": ""
        },
        "copperThickness": {
          "min": 5,
          "max": null,
          "unit": "μm"
        },
        "coverHeight": {
          "min": 0.01,
          "max": null,
          "unit": "mm"
        },
        "coverDielectric": {
          "min": 1,
          "max": null,
          "unit": ""
        },
        "frequency": {
          "min": 0.001,
          "max": 300,
          "unit": "GHz"
        }
      },
      "outside": [],
      "model": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "traceType": 0,
      "traceWidth": 1.8,
      "substrateHeight": 0.2,
      "dielectricConst": 4.4,
      "copperThickness": 35,
      "coverHeight": 0.1,
      "coverDielectric": 3.5,
      "frequency": 2.4
    },
    "seed": null,
    "elapsedSeconds": 0.003
  }
}

{
  "detail": [
    {
      "type": "float_parsing",
      "loc": [
        "body",
        "inputs",
        "traceType"
      ],
      "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 pagePCB Controlled Impedance Calculator, at these values