Documentation sections

Critical Trace Length Calculator

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

Slug critical-trace-length. Find the maximum PCB trace length before transmission-line effects matter. Compares your trace against Bogatin's 20% and 3× rise-time criteria and reports propagation delay.

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 critical-trace-length
Name	Unit	Range	Default	Description
riseTimenumber	ps	at least 1	500	Signal Rise Time
traceTypenumber	none	0 … 1	0	Trace TypeOne of: 0 Microstrip (outer layer) · 1 Stripline (inner layer)
dielectricConstnumber	none	at least 1	4.2	Microstrip εr_eff assumes W/h = 2; other aspect ratios shift the delay by a few percent
traceLengthnumber	mm	at least 0	50	Actual Trace Length
criterionnumber	none	0 … 1	0	Conservative is Bogatin's 20% rule (reflections virtually invisible); permissive is his t_r > 3·t_d thresholdOne of: 0 Conservative (t_d < 0.2·t_r) · 1 Permissive (t_r > 3·t_d)

Outputs

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

Outputs of critical-trace-length
Field	Unit	Description
criticalLengthheadline	mm	Critical Trace Length
criticalLengthPermissive	mm	Critical Length (permissive rule)
lengthRatio	×	Actual ÷ Critical Length
traceDelay	ps	One-Way Trace Delay
propagationDelay	ps/mm	Propagation Delay
effectiveDielectric	none	Effective Dielectric Constant

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": "critical-trace-length",
    "inputs": {
      "riseTime": 500,
      "traceType": 0,
      "dielectricConst": 4.2,
      "traceLength": 50,
      "criterion": 0
    }
  }'

import rftools

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

result = client.calculate("critical-trace-length", {
    "riseTime": 500,
    "traceType": 0,
    "dielectricConst": 4.2,
    "traceLength": 50,
    "criterion": 0,
})
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": "critical-trace-length",
    "inputs": {
      "riseTime": 500,
      "traceType": 0,
      "dielectricConst": 4.2,
      "traceLength": 50,
      "criterion": 0
    }
  }),
});
console.log(response.status, await response.json());

package main

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

func main() {
	body := []byte(`{
	  "slug": "critical-trace-length",
	  "inputs": {
	    "riseTime": 500,
	    "traceType": 0,
	    "dielectricConst": 4.2,
	    "traceLength": 50,
	    "criterion": 0
	  }
	}`)
	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": "critical-trace-length",
  "values": {
    "criticalLength": 16.746501338594538,
    "criticalLengthPermissive": 27.91083556432423,
    "propagationDelay": 5.971396531019689,
    "traceDelay": 298.56982655098443,
    "lengthRatio": 2.9856982655098445,
    "effectiveDielectric": 3.2047431568147635
  },
  "warnings": [
    "Trace is 2.99× the critical length — terminate it and route it as a controlled-impedance transmission line"
  ],
  "errors": [],
  "provenance": {
    "method": "calculator:critical-trace-length",
    "version": "api@3f2c1a9b04de",
    "formulaRef": "E. Bogatin, \"Signal and Power Integrity — Simplified\", 2nd ed. (20% rule); E. Bogatin, \"The Critical Length of a Transmission Line\", Polar Instruments",
    "assumptions": [],
    "validRange": {
      "status": "inside",
      "bounds": {
        "riseTime": {
          "min": 1,
          "max": null,
          "unit": "ps"
        },
        "traceType": {
          "min": 0,
          "max": 1,
          "unit": ""
        },
        "dielectricConst": {
          "min": 1,
          "max": null,
          "unit": ""
        },
        "traceLength": {
          "min": 0,
          "max": null,
          "unit": "mm"
        },
        "criterion": {
          "min": 0,
          "max": 1,
          "unit": ""
        }
      },
      "outside": [],
      "model": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "riseTime": 500,
      "traceType": 0,
      "dielectricConst": 4.2,
      "traceLength": 50,
      "criterion": 0
    },
    "seed": null,
    "elapsedSeconds": 0.003
  }
}

{
  "detail": [
    {
      "type": "float_parsing",
      "loc": [
        "body",
        "inputs",
        "riseTime"
      ],
      "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 pageCritical Trace Length Calculator, at these values