Documentation sections

Microstrip Impedance Calculator

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

Slug microstrip-impedance. Calculate microstrip impedance using Hammerstad-Jensen equations. Get Z0, effective dielectric constant, and propagation delay for PCB trace design. 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 microstrip-impedance
Name	Unit	Range	Default	Description
traceWidthnumber	mm	0.01 … 50	1.2	Width of the copper trace on top of the substrate
substrateHeightnumber	mm	0.05 … 10	1.6	Dielectric thickness between trace and ground plane
dielectricConstantnumber	none	1 … 100	4.2	Relative permittivity of the substrate material
copperThicknessnumber	μm	1 … 200	35	Thickness of the copper trace
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 microstrip-impedance
Field	Unit	Description
impedanceheadline	Ω	Characteristic Impedance
effectiveDielectric	none	Effective Dielectric
propagationDelay	ps/mm	Propagation Delay
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": "microstrip-impedance",
    "inputs": {
      "traceWidth": 1.2,
      "substrateHeight": 1.6,
      "dielectricConstant": 4.2,
      "copperThickness": 35,
      "frequency": 2.4
    }
  }'

import rftools

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

result = client.calculate("microstrip-impedance", {
    "traceWidth": 1.2,
    "substrateHeight": 1.6,
    "dielectricConstant": 4.2,
    "copperThickness": 35,
    "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": "microstrip-impedance",
    "inputs": {
      "traceWidth": 1.2,
      "substrateHeight": 1.6,
      "dielectricConstant": 4.2,
      "copperThickness": 35,
      "frequency": 2.4
    }
  }),
});
console.log(response.status, await response.json());

package main

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

func main() {
	body := []byte(`{
	  "slug": "microstrip-impedance",
	  "inputs": {
	    "traceWidth": 1.2,
	    "substrateHeight": 1.6,
	    "dielectricConstant": 4.2,
	    "copperThickness": 35,
	    "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": "microstrip-impedance",
  "values": {
    "impedance": 80.898294407455,
    "effectiveDielectric": 2.9968834251363994,
    "propagationDelay": 5.774497823156724,
    "guidedWavelength": 72.15634665161048,
    "quarterWavelength": 18.03908666290262
  },
  "warnings": [],
  "errors": [],
  "provenance": {
    "method": "calculator:microstrip-impedance",
    "version": "api@3f2c1a9b04de",
    "formulaRef": "Hammerstad & Jensen (1980); Wadell, \"Transmission Line Design Handbook\" 1991",
    "assumptions": [
      {
        "code": "quasi-static",
        "text": "Quasi-static: Z₀ and εeff are the Hammerstad–Jensen static values, with no frequency dependence; dispersion is not modelled."
      },
      {
        "code": "bare-microstrip",
        "text": "Bare microstrip: air above the trace, with no soldermask or other cover layer."
      },
      {
        "code": "homogeneous-substrate",
        "text": "One homogeneous, isotropic, non-magnetic substrate of relative permittivity εr over a solid ground plane, both extending well beyond the trace."
      },
      {
        "code": "thickness-effective-width",
        "text": "Copper thickness t is modelled as an effective width increase Δw = (t/π)(1 + ln(2h/t)), applied to both Z₀ and εeff."
      },
      {
        "code": "lossless",
        "text": "Lossless: conductor and dielectric losses are neglected, so Z₀ is real and the propagation delay is √εeff/c."
      }
    ],
    "validRange": {
      "status": "inside",
      "bounds": {
        "traceWidth": {
          "min": 0.01,
          "max": 50,
          "unit": "mm"
        },
        "substrateHeight": {
          "min": 0.05,
          "max": 10,
          "unit": "mm"
        },
        "dielectricConstant": {
          "min": 1,
          "max": 100,
          "unit": ""
        },
        "copperThickness": {
          "min": 1,
          "max": 200,
          "unit": "μm"
        },
        "frequency": {
          "min": 0.001,
          "max": 300,
          "unit": "GHz"
        }
      },
      "outside": [],
      "model": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "traceWidth": 1.2,
      "substrateHeight": 1.6,
      "dielectricConstant": 4.2,
      "copperThickness": 35,
      "frequency": 2.4
    },
    "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 pageMicrostrip Impedance Calculator, at these values