Documentation sections

4-Layer PCB Stackup Impedance Calculator

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

Slug stackup-builder. Calculate trace widths for a target impedance on a 4-layer PCB stackup: microstrip on L1 over the L2 plane, stripline on L3 between the L2 and L4 planes. Enter the dielectric heights, dielectric constant and copper weight for 50 ohm or a custom Z0. 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 stackup-builder
Name	Unit	Range	Default	Description
targetImpedancenumber	Ω	25 … 150	50	Target characteristic impedance for trace width calculation
dielectricH1number	mm	0.05 … 2	0.2	Prepreg thickness between L1 (top) and L2
dielectricH2number	mm	0.1 … 5	1.2	Core thickness between L2 and L3
dielectricH3number	mm	0.05 … 2	0.2	Prepreg thickness between L3 and L4 (bottom)
copperWeightnumber	oz	0.5 … 3	1	Copper foil weight (1 oz = 35 μm)
dielectricConstantnumber	none	2 … 12	4.2	Relative permittivity of the board material at design frequency

Retired, still accepted and ignored with a warning: layerCount (Layer Count, 2026-10-12).

Outputs

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

Outputs of stackup-builder
Field	Unit	Description
traceWidthL1mmheadline	mm	Trace width for target impedance on L1 (surface microstrip over L2), solved from Hammerstad-Jensen with the copper thickness
traceWidthL3mm	mm	Trace width for target impedance on L3 (stripline between the L2 and L4 planes, H2 and H3 away), solved from the asymmetric-stripline model
achievedImpedanceL1	Ω	Impedance of the L1 trace at the reported width (Hammerstad-Jensen, with t)
achievedImpedanceL3	Ω	Impedance of the L3 trace at the reported width (asymmetric stripline between L2 and L4)
totalThicknessMm	mm	Three dielectric layers and four copper layers: H1 + H2 + H3 + 4·t

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": "stackup-builder",
    "inputs": {
      "targetImpedance": 50,
      "dielectricH1": 0.2,
      "dielectricH2": 1.2,
      "dielectricH3": 0.2,
      "copperWeight": 1,
      "dielectricConstant": 4.2
    }
  }'

import rftools

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

result = client.calculate("stackup-builder", {
    "targetImpedance": 50,
    "dielectricH1": 0.2,
    "dielectricH2": 1.2,
    "dielectricH3": 0.2,
    "copperWeight": 1,
    "dielectricConstant": 4.2,
})
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": "stackup-builder",
    "inputs": {
      "targetImpedance": 50,
      "dielectricH1": 0.2,
      "dielectricH2": 1.2,
      "dielectricH3": 0.2,
      "copperWeight": 1,
      "dielectricConstant": 4.2
    }
  }),
});
console.log(response.status, await response.json());

package main

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

func main() {
	body := []byte(`{
	  "slug": "stackup-builder",
	  "inputs": {
	    "targetImpedance": 50,
	    "dielectricH1": 0.2,
	    "dielectricH2": 1.2,
	    "dielectricH3": 0.2,
	    "copperWeight": 1,
	    "dielectricConstant": 4.2
	  }
	}`)
	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": "stackup-builder",
  "values": {
    "traceWidthL1mm": 0.35795746280812923,
    "traceWidthL3mm": 0.2587100647562412,
    "achievedImpedanceL1": 49.99999999999937,
    "achievedImpedanceL3": 49.99999999999934,
    "totalThicknessMm": 1.7399999999999998
  },
  "warnings": [],
  "errors": [],
  "provenance": {
    "method": "calculator:stackup-builder",
    "version": "api@3f2c1a9b04de",
    "formulaRef": "Hammerstad & Jensen (1980); Cohn (1954), offset stripline; IPC-2141A",
    "assumptions": [],
    "validRange": {
      "status": "inside",
      "bounds": {
        "targetImpedance": {
          "min": 25,
          "max": 150,
          "unit": "Ω"
        },
        "dielectricH1": {
          "min": 0.05,
          "max": 2,
          "unit": "mm"
        },
        "dielectricH2": {
          "min": 0.1,
          "max": 5,
          "unit": "mm"
        },
        "dielectricH3": {
          "min": 0.05,
          "max": 2,
          "unit": "mm"
        },
        "copperWeight": {
          "min": 0.5,
          "max": 3,
          "unit": "oz"
        },
        "dielectricConstant": {
          "min": 2,
          "max": 12,
          "unit": ""
        }
      },
      "outside": [],
      "model": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "targetImpedance": 50,
      "dielectricH1": 0.2,
      "dielectricH2": 1.2,
      "dielectricH3": 0.2,
      "copperWeight": 1,
      "dielectricConstant": 4.2
    },
    "seed": null,
    "elapsedSeconds": 0.003
  }
}

{
  "detail": [
    {
      "type": "float_parsing",
      "loc": [
        "body",
        "inputs",
        "targetImpedance"
      ],
      "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 page4-Layer PCB Stackup Impedance Calculator, at these values