Documentation sections

Dual Stripline Impedance Calculator

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

Slug dual-stripline. Calculate impedance for both signal layers of a dual stripline (two routing layers sharing one plane pair), plus the broadside crosstalk penalty when traces overlap.

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 dual-stripline
Name	Unit	Range	Default	Description
traceWidthnumber	mm	0.02 … 10	0.13	Trace Width
planeSpacingnumber	mm	at least 0.05	0.8	Total dielectric height between the two reference planes
layer1Offsetnumber	mm	0.01 … 5	0.2	Layer 1 to Upper Plane
layerSeparationnumber	mm	at least 0.01	0.33	Separation Between Signal Layers
copperThicknessnumber	μm	at least 5	35	Copper Thickness
dielectricConstnumber	none	at least 1	4.2	Dielectric Constant
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 dual-stripline
Field	Unit	Description
impedanceLayer1headline	Ω	Layer 1 Impedance
impedanceLayer2	Ω	Layer 2 Impedance
layer2Offset	mm	Layer 2 to Lower Plane
broadsideCoupling	%	Backward-crosstalk coefficient if the two layers run directly over each other
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": "dual-stripline",
    "inputs": {
      "traceWidth": 0.13,
      "planeSpacing": 0.8,
      "layer1Offset": 0.2,
      "layerSeparation": 0.33,
      "copperThickness": 35,
      "dielectricConst": 4.2,
      "frequency": 2.4
    }
  }'

import rftools

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

result = client.calculate("dual-stripline", {
    "traceWidth": 0.13,
    "planeSpacing": 0.8,
    "layer1Offset": 0.2,
    "layerSeparation": 0.33,
    "copperThickness": 35,
    "dielectricConst": 4.2,
    "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": "dual-stripline",
    "inputs": {
      "traceWidth": 0.13,
      "planeSpacing": 0.8,
      "layer1Offset": 0.2,
      "layerSeparation": 0.33,
      "copperThickness": 35,
      "dielectricConst": 4.2,
      "frequency": 2.4
    }
  }),
});
console.log(response.status, await response.json());

package main

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

func main() {
	body := []byte(`{
	  "slug": "dual-stripline",
	  "inputs": {
	    "traceWidth": 0.13,
	    "planeSpacing": 0.8,
	    "layer1Offset": 0.2,
	    "layerSeparation": 0.33,
	    "copperThickness": 35,
	    "dielectricConst": 4.2,
	    "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": "dual-stripline",
  "values": {
    "impedanceLayer1": 60.514048655897255,
    "impedanceLayer2": 60.514048655897255,
    "layer2Offset": 0.20000000000000007,
    "propagationDelay": 6.83602972157465,
    "broadsideCoupling": 21.547607999508887,
    "guidedWavelength": 60.95155867325418,
    "quarterWavelength": 15.237889668313546
  },
  "warnings": [
    "Broadside coupling is 21.5% if the layers overlap — route the two signal layers orthogonally"
  ],
  "errors": [],
  "provenance": {
    "method": "calculator:dual-stripline",
    "version": "api@3f2c1a9b04de",
    "formulaRef": "S. B. Cohn, “Characteristic Impedance of the Shielded-Strip Transmission Line”, IRE Trans. MTT-2, 1954 (each layer, with the off-centre fringing fitted to rftools.io’s 2-D method-of-moments solver); IPC-2141A; E. Bogatin, \"Signal and Power Integrity — Simplified\", 3rd ed., ch. 11",
    "assumptions": [],
    "validRange": {
      "status": "inside",
      "bounds": {
        "traceWidth": {
          "min": 0.02,
          "max": 10,
          "unit": "mm"
        },
        "planeSpacing": {
          "min": 0.05,
          "max": null,
          "unit": "mm"
        },
        "layer1Offset": {
          "min": 0.01,
          "max": 5,
          "unit": "mm"
        },
        "layerSeparation": {
          "min": 0.01,
          "max": null,
          "unit": "mm"
        },
        "copperThickness": {
          "min": 5,
          "max": null,
          "unit": "μm"
        },
        "dielectricConst": {
          "min": 1,
          "max": null,
          "unit": ""
        },
        "frequency": {
          "min": 0.001,
          "max": 300,
          "unit": "GHz"
        }
      },
      "outside": [],
      "model": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "traceWidth": 0.13,
      "planeSpacing": 0.8,
      "layer1Offset": 0.2,
      "layerSeparation": 0.33,
      "copperThickness": 35,
      "dielectricConst": 4.2,
      "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 pageDual Stripline Impedance Calculator, at these values