Documentation sections

Effective Dielectric Constant Calculator (Microstrip)

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

Slug effective-dielectric-constant. Calculate microstrip effective dielectric constant εeff with Hammerstad–Jensen, plus Getsinger dispersion, propagation velocity, delay in ps/mm and ps/inch, and guided wavelength.

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 effective-dielectric-constant
Name	Unit	Range	Default	Description
traceWidthnumber	mm	0.01 … 50	0.3	Trace Width
dielectricHeightnumber	mm	0.01 … 10	0.2	Thickness of dielectric between the trace and the reference plane below it.
dielectricConstnumber	none	1 … 20	4.3	Dielectric Constant
copperThicknessnumber	µm	1 … 400	35	Copper Thickness
frequencynumber	GHz	0.001 … 110	1	Used for the dispersion correction and the guided wavelength.

Outputs

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

Outputs of effective-dielectric-constant
Field	Unit	Description
erEffheadline	none	Effective Dielectric Constant (static)
erEffAtFreq	none	εeff with Dispersion
dispersionPercent	%	Dispersion Shift
fillingFactor	none	Filling Factor
z0Ohm	Ω	Characteristic Impedance
velocityPercentC	% of c	Propagation Velocity
propDelayPsPerMm	ps/mm	Propagation Delay
propDelayPsPerInch	ps/inch	Propagation Delay
wavelengthMm	mm	Guided Wavelength
quarterWaveMm	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": "effective-dielectric-constant",
    "inputs": {
      "traceWidth": 0.3,
      "dielectricHeight": 0.2,
      "dielectricConst": 4.3,
      "copperThickness": 35,
      "frequency": 1
    }
  }'

import rftools

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

result = client.calculate("effective-dielectric-constant", {
    "traceWidth": 0.3,
    "dielectricHeight": 0.2,
    "dielectricConst": 4.3,
    "copperThickness": 35,
    "frequency": 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": "effective-dielectric-constant",
    "inputs": {
      "traceWidth": 0.3,
      "dielectricHeight": 0.2,
      "dielectricConst": 4.3,
      "copperThickness": 35,
      "frequency": 1
    }
  }),
});
console.log(response.status, await response.json());

package main

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

func main() {
	body := []byte(`{
	  "slug": "effective-dielectric-constant",
	  "inputs": {
	    "traceWidth": 0.3,
	    "dielectricHeight": 0.2,
	    "dielectricConst": 4.3,
	    "copperThickness": 35,
	    "frequency": 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": "effective-dielectric-constant",
  "values": {
    "erEff": 3.2296814465543817,
    "erEffAtFreq": 3.229781202715551,
    "fillingFactor": 0.6756610444104187,
    "z0Ohm": 54.330730981424914,
    "velocityPercentC": 55.64423238564186,
    "propDelayPsPerMm": 5.994585258834897,
    "propDelayPsPerInch": 152.26246557440638,
    "wavelengthMm": 166.8146357966372,
    "quarterWaveMm": 41.7036589491593,
    "dispersionPercent": 0.003088730663382681
  },
  "warnings": [],
  "errors": [],
  "provenance": {
    "method": "calculator:effective-dielectric-constant",
    "version": "api@3f2c1a9b04de",
    "formulaRef": "E. Hammerstad & Ø. Jensen, \"Accurate Models for Microstrip Computer-Aided Design\", IEEE MTT-S 1980; W. J. Getsinger, \"Microstrip Dispersion Model\", IEEE Trans. MTT-21, 1973",
    "assumptions": [],
    "validRange": {
      "status": "inside",
      "bounds": {
        "traceWidth": {
          "min": 0.01,
          "max": 50,
          "unit": "mm"
        },
        "dielectricHeight": {
          "min": 0.01,
          "max": 10,
          "unit": "mm"
        },
        "dielectricConst": {
          "min": 1,
          "max": 20,
          "unit": ""
        },
        "copperThickness": {
          "min": 1,
          "max": 400,
          "unit": "µm"
        },
        "frequency": {
          "min": 0.001,
          "max": 110,
          "unit": "GHz"
        }
      },
      "outside": [],
      "model": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "traceWidth": 0.3,
      "dielectricHeight": 0.2,
      "dielectricConst": 4.3,
      "copperThickness": 35,
      "frequency": 1
    },
    "seed": null,
    "elapsedSeconds": 0.003
  }
}

{
  "detail": [
    {
      "type": "float_parsing",
      "loc": [
        "body",
        "inputs",
        "traceWidth"
      ],
      "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 pageEffective Dielectric Constant Calculator (Microstrip), at these values