Documentation sections

Via Step Response & Rise-Time Degradation Calculator

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

Slug via-step-response. Model a PCB via as a lumped LC discontinuity: excess capacitance and inductance, self-resonant frequency, 10–90 % rise-time degradation, and peak reflection for a given edge rate.

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 via-step-response
Name	Unit	Range	Default	Description
viaLengthnumber	mm	0.05 … 6.5	1.6	Barrel length the signal actually traverses. Back-drilled vias should use the remaining length, not the board thickness.
drillDiameternumber	mm	0.05 … 2	0.3	Drill Diameter
padDiameternumber	mm	0.1 … 3	0.6	Pad Diameter
antipadDiameternumber	mm	0.15 … 6	1	Plane clearance around the via. Enlarging it is the cheapest way to cut via capacitance.
dielectricConstnumber	none	1 … 20	4.3	Dielectric Constant
lineImpedancenumber	Ω	10 … 200	50	Line Impedance
riseTimenumber	ps	1 … 10000	400	Input Rise Time (10–90 %)
slowdownBudgetnumber	%	0.1 … 100	10	How much the via may slow the edge, as a share of the input rise time; the calculator warned at 10 % before it could be set

Outputs

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

Outputs of via-step-response
Field	Unit	Description
riseTimeOutPsheadline	ps	Output Rise Time
riseTimeDegradationPs	ps	Rise-Time Degradation
degradationPercent	%	Edge Slowdown
reflectionPercent	%	Peak Reflection
inductanceNH	nH	Via Inductance
capacitancePF	pF	Via Capacitance
excessCapacitancePF	pF	Excess Capacitance
excessInductanceNH	nH	Excess Inductance
zViaOhm	Ω	Via Impedance √(L/C)
fResonanceGHz	GHz	LC Self-Resonance
kneeFrequencyGHz	GHz	Signal Knee Frequency

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": "via-step-response",
    "inputs": {
      "viaLength": 1.6,
      "drillDiameter": 0.3,
      "padDiameter": 0.6,
      "antipadDiameter": 1,
      "dielectricConst": 4.3,
      "lineImpedance": 50,
      "riseTime": 400,
      "slowdownBudget": 10
    }
  }'

import rftools

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

result = client.calculate("via-step-response", {
    "viaLength": 1.6,
    "drillDiameter": 0.3,
    "padDiameter": 0.6,
    "antipadDiameter": 1,
    "dielectricConst": 4.3,
    "lineImpedance": 50,
    "riseTime": 400,
    "slowdownBudget": 10,
})
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": "via-step-response",
    "inputs": {
      "viaLength": 1.6,
      "drillDiameter": 0.3,
      "padDiameter": 0.6,
      "antipadDiameter": 1,
      "dielectricConst": 4.3,
      "lineImpedance": 50,
      "riseTime": 400,
      "slowdownBudget": 10
    }
  }),
});
console.log(response.status, await response.json());

package main

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

func main() {
	body := []byte(`{
	  "slug": "via-step-response",
	  "inputs": {
	    "viaLength": 1.6,
	    "drillDiameter": 0.3,
	    "padDiameter": 0.6,
	    "antipadDiameter": 1,
	    "dielectricConst": 4.3,
	    "lineImpedance": 50,
	    "riseTime": 400,
	    "slowdownBudget": 10
	  }
	}`)
	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": "via-step-response",
  "values": {
    "riseTimeOutPs": 400.05088896457,
    "riseTimeDegradationPs": 6.380733605371386,
    "degradationPercent": 0.01272224114250431,
    "inductanceNH": 1.1392866543013003,
    "capacitancePF": 0.5717279999999999,
    "zViaOhm": 44.63975208617234,
    "fResonanceGHz": 6.236040048470675,
    "excessCapacitancePF": 0.11601333827947972,
    "excessInductanceNH": -0.2900333456986994,
    "reflectionPercent": 0.7250833642467482,
    "kneeFrequencyGHz": 1.25
  },
  "warnings": [],
  "errors": [],
  "provenance": {
    "method": "calculator:via-step-response",
    "version": "api@3f2c1a9b04de",
    "formulaRef": "H. Johnson & M. Graham, \"High-Speed Digital Design\", Prentice Hall 1993; IPC-2141A (via capacitance)",
    "assumptions": [],
    "validRange": {
      "status": "inside",
      "bounds": {
        "viaLength": {
          "min": 0.05,
          "max": 6.5,
          "unit": "mm"
        },
        "drillDiameter": {
          "min": 0.05,
          "max": 2,
          "unit": "mm"
        },
        "padDiameter": {
          "min": 0.1,
          "max": 3,
          "unit": "mm"
        },
        "antipadDiameter": {
          "min": 0.15,
          "max": 6,
          "unit": "mm"
        },
        "dielectricConst": {
          "min": 1,
          "max": 20,
          "unit": ""
        },
        "lineImpedance": {
          "min": 10,
          "max": 200,
          "unit": "Ω"
        },
        "riseTime": {
          "min": 1,
          "max": 10000,
          "unit": "ps"
        },
        "slowdownBudget": {
          "min": 0.1,
          "max": 100,
          "unit": "%"
        }
      },
      "outside": [],
      "model": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "viaLength": 1.6,
      "drillDiameter": 0.3,
      "padDiameter": 0.6,
      "antipadDiameter": 1,
      "dielectricConst": 4.3,
      "lineImpedance": 50,
      "riseTime": 400,
      "slowdownBudget": 10
    },
    "seed": null,
    "elapsedSeconds": 0.003
  }
}

{
  "detail": [
    {
      "type": "float_parsing",
      "loc": [
        "body",
        "inputs",
        "viaLength"
      ],
      "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 pageVia Step Response & Rise-Time Degradation Calculator, at these values