Documentation sections

PCB Via Calculator

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

Slug via-calculator. Calculate PCB via impedance, capacitance, inductance, and current capacity. Get aspect ratio and DFM warnings for through-hole and blind vias. 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 via-calculator
Name	Unit	Range	Default	Description
viaDiameternumber	mm	0.1 … 6	0.3	Finished drill diameter of the via hole
padDiameternumber	mm	0.2 … 10	0.6	Copper pad diameter surrounding the via
antipadDiameternumber	mm	0.25 … 12	1	Plane clearance around the via. Sets both the coaxial outer conductor and the far plate of the via capacitance - enlarging it is the cheapest way to cut via capacitance.
boardThicknessnumber	mm	0.1 … 10	1.6	Total PCB thickness (for aspect ratio and inductance)
dielectricConstantnumber	none	1 … 20	4.2	Relative permittivity of the board material
copperThicknessnumber	μm	5 … 100	25	Plated copper wall thickness inside the via barrel

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

Outputs

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

Outputs of via-calculator
Field	Unit	Description
impedanceheadline	Ω	Coaxial approximation with the barrel as inner conductor and the antipad as outer conductor
capacitancePF	pF	Pad-to-antipad capacitance (IPC-2141A). Grows sharply as the plane clearance closes in on the pad.
inductanceNH	nH	Parasitic inductance of the via (Wheeler)
aspectRatio	none	Board thickness / drill diameter. Keep ≤ 10:1 for reliable plating.
currentCapacityA	A	IPC-2221 at 10°C rise on the plated annulus A = pit(d-t). Uses the external-conductor constant k = 0.048.

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-calculator",
    "inputs": {
      "viaDiameter": 0.3,
      "padDiameter": 0.6,
      "antipadDiameter": 1,
      "boardThickness": 1.6,
      "dielectricConstant": 4.2,
      "copperThickness": 25
    }
  }'

import rftools

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

result = client.calculate("via-calculator", {
    "viaDiameter": 0.3,
    "padDiameter": 0.6,
    "antipadDiameter": 1,
    "boardThickness": 1.6,
    "dielectricConstant": 4.2,
    "copperThickness": 25,
})
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-calculator",
    "inputs": {
      "viaDiameter": 0.3,
      "padDiameter": 0.6,
      "antipadDiameter": 1,
      "boardThickness": 1.6,
      "dielectricConstant": 4.2,
      "copperThickness": 25
    }
  }),
});
console.log(response.status, await response.json());

package main

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

func main() {
	body := []byte(`{
	  "slug": "via-calculator",
	  "inputs": {
	    "viaDiameter": 0.3,
	    "padDiameter": 0.6,
	    "antipadDiameter": 1,
	    "boardThickness": 1.6,
	    "dielectricConstant": 4.2,
	    "copperThickness": 25
	  }
	}`)
	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-calculator",
  "values": {
    "impedance": 35.22432915063511,
    "capacitancePF": 0.558432,
    "inductanceNH": 1.1392866543013003,
    "aspectRatio": 5.333333333333334,
    "currentCapacityA": 1.6853446055708448
  },
  "warnings": [],
  "errors": [],
  "provenance": {
    "method": "calculator:via-calculator",
    "version": "api@3f2c1a9b04de",
    "formulaRef": "IPC-2141A; IPC-2221A (2003) Figure 6-4, chart A (the chart of IPC-2221, 1998), through its published fit, for the current capacity; Howard Johnson \"High-Speed Signal Propagation\"",
    "assumptions": [
      {
        "code": "through-via",
        "text": "A plated through via in a board of one dielectric constant: its height is the full board thickness; blind, buried and back-drilled vias are not modelled."
      },
      {
        "code": "coaxial-impedance",
        "text": "Impedance treats the drilled barrel and the antipad edge as a coaxial line, Z = (η₀/2π)·ln(D_antipad/d)/√εr with η₀/2π = 59.9585 Ω; the pads, the unused stub and return vias are not modelled."
      },
      {
        "code": "ipc-2141-capacitance",
        "text": "Capacitance uses the IPC-2141A pad-to-antipad form C = 0.0554·εr·T·D/(D_antipad − D) pF, dimensions in mm."
      },
      {
        "code": "isolated-wire-inductance",
        "text": "Inductance is that of an isolated round conductor of length T and diameter d, L = 0.2·T·(ln(4T/d) + 0.5) nH; the return path through nearby planes and vias, which sets the loop inductance, is not modelled."
      },
      {
        "code": "lumped",
        "text": "Capacitance and inductance are lumped values, valid while the via is short compared with a wavelength."
      },
      {
        "code": "ipc-2221-current",
        "text": "Current capacity applies the IPC-2221 external-layer fit (k = 0.048) at a fixed 10 °C rise to the plated annulus A = π·t·(d − t)."
      },
      {
        "code": "signal-layer-ignored",
        "text": "The signal-layer input does not change any output."
      }
    ],
    "validRange": {
      "status": "inside",
      "bounds": {
        "viaDiameter": {
          "min": 0.1,
          "max": 6,
          "unit": "mm"
        },
        "padDiameter": {
          "min": 0.2,
          "max": 10,
          "unit": "mm"
        },
        "antipadDiameter": {
          "min": 0.25,
          "max": 12,
          "unit": "mm"
        },
        "boardThickness": {
          "min": 0.1,
          "max": 10,
          "unit": "mm"
        },
        "dielectricConstant": {
          "min": 1,
          "max": 20,
          "unit": ""
        },
        "copperThickness": {
          "min": 5,
          "max": 100,
          "unit": "μm"
        }
      },
      "outside": [],
      "model": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "viaDiameter": 0.3,
      "padDiameter": 0.6,
      "antipadDiameter": 1,
      "boardThickness": 1.6,
      "dielectricConstant": 4.2,
      "copperThickness": 25
    },
    "seed": null,
    "elapsedSeconds": 0.003
  }
}

{
  "detail": [
    {
      "type": "float_parsing",
      "loc": [
        "body",
        "inputs",
        "viaDiameter"
      ],
      "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 pagePCB Via Calculator, at these values