Documentation sections

Q Factor Calculator for Inductors & Capacitors

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

Slug q-factor. Calculate quality factor (Q) for inductors and capacitors from ESR and frequency. Determine resonant bandwidth and half-power frequencies. 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 q-factor
Name	Unit	Range	Default	Description
componentTypenumber	none	0 … 1	0	Component TypeOne of: 0 Inductor · 1 Capacitor
valuenumber	μH / nF	at least 0.001	10	Inductance / Capacitance
esrnumber	Ω	at least 0.001	0.5	Equivalent Series Resistance from datasheet
frequencynumber	MHz	at least 0.001	10	Frequency
targetQnumber	none	1 … 10000	50	The quality factor the component must reach, for example from a filter design. The status fails below 10.

Outputs

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

Outputs of q-factor
Field	Unit	Description
qFactorheadline	none	Q Factor
reactance	Ω	Reactance
bandwidth	kHz	Bandwidth
halfPowerFreqLow	MHz	Lower Half-Power Frequency
halfPowerFreqHigh	MHz	Upper Half-Power 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": "q-factor",
    "inputs": {
      "componentType": 0,
      "value": 10,
      "esr": 0.5,
      "frequency": 10,
      "targetQ": 50
    }
  }'

import rftools

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

result = client.calculate("q-factor", {
    "componentType": 0,
    "value": 10,
    "esr": 0.5,
    "frequency": 10,
    "targetQ": 50,
})
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": "q-factor",
    "inputs": {
      "componentType": 0,
      "value": 10,
      "esr": 0.5,
      "frequency": 10,
      "targetQ": 50
    }
  }),
});
console.log(response.status, await response.json());

package main

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

func main() {
	body := []byte(`{
	  "slug": "q-factor",
	  "inputs": {
	    "componentType": 0,
	    "value": 10,
	    "esr": 0.5,
	    "frequency": 10,
	    "targetQ": 50
	  }
	}`)
	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": "q-factor",
  "values": {
    "qFactor": 1256.6370614359173,
    "reactance": 628.3185307179587,
    "bandwidth": 7.957747154594767,
    "halfPowerFreqLow": 9.996021126422702,
    "halfPowerFreqHigh": 10.003978873577298
  },
  "warnings": [],
  "errors": [],
  "provenance": {
    "method": "calculator:q-factor",
    "version": "api@3f2c1a9b04de",
    "formulaRef": "Microwave Engineering, 4th ed., David M. Pozar (2011), Chapter 6.1 — Quality factor and resonator loss",
    "assumptions": [],
    "validRange": {
      "status": "inside",
      "bounds": {
        "componentType": {
          "min": 0,
          "max": 1,
          "unit": ""
        },
        "value": {
          "min": 0.001,
          "max": null,
          "unit": "μH / nF"
        },
        "esr": {
          "min": 0.001,
          "max": null,
          "unit": "Ω"
        },
        "frequency": {
          "min": 0.001,
          "max": null,
          "unit": "MHz"
        },
        "targetQ": {
          "min": 1,
          "max": 10000,
          "unit": ""
        }
      },
      "outside": [],
      "model": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "componentType": 0,
      "value": 10,
      "esr": 0.5,
      "frequency": 10,
      "targetQ": 50
    },
    "seed": null,
    "elapsedSeconds": 0.003
  }
}

{
  "detail": [
    {
      "type": "float_parsing",
      "loc": [
        "body",
        "inputs",
        "componentType"
      ],
      "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 pageQ Factor Calculator for Inductors & Capacitors, at these values