Documentation sections

LC EMI Filter Design Calculator

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

Slug emi-filter-lc. Design LC EMI filters for conducted emissions compliance. Calculate inductance, capacitance, cutoff frequency, and attenuation for CISPR 22/32 and FCC Part 15. 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 emi-filter-lc
Name	Unit	Range	Default	Description
fcnumber	kHz	0.1 … 100000	150	Filter -3dB cutoff frequency (150 kHz = CISPR conducted emissions limit start)
attenuationNeedednumber	dB	6 … 120	40	The attenuation needed at ten times the cutoff frequency; it sets the filter order, n = ⌈A/20⌉
zSourcenumber	Ω	0.1 … 10000	50	Source impedance (50Ω for LISN test environments)
zLoadnumber	Ω	0.1 … 10000	50	Load impedance
eSeriesnumber	none	0 … 96	0	Rounds every part to the nearest value in the series; the response and the export are the rounded design's.One of: 0 Ideal values · 12 E12 (10%) · 24 E24 (5%) · 48 E48 (2%) · 96 E96 (1%)

Outputs

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

Outputs of emi-filter-lc
Field	Unit	Description
inductanceheadline	μH	Filter inductor value per stage; rounded to the part series when one is chosen
capacitance	μF	Filter capacitor value per stage; rounded to the part series when one is chosen
filterOrder	none	Number of LC stages required to achieve the specified attenuation
actualAttenuation	dB	Actual attenuation at 10× the cutoff frequency (one decade into stop band)
impedanceMatching	Ω	Geometric mean impedance for filter matching
insertionLossBuilt	dB	What the exported pi filter (C1, L, C2) with these part values does at 10× the cutoff frequency between the source and load impedances: the circuit's insertion loss, solved as a ladder, not the per-order estimate above

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": "emi-filter-lc",
    "inputs": {
      "fc": 150,
      "attenuationNeeded": 40,
      "zSource": 50,
      "zLoad": 50,
      "eSeries": 0
    }
  }'

import rftools

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

result = client.calculate("emi-filter-lc", {
    "fc": 150,
    "attenuationNeeded": 40,
    "zSource": 50,
    "zLoad": 50,
    "eSeries": 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": "emi-filter-lc",
    "inputs": {
      "fc": 150,
      "attenuationNeeded": 40,
      "zSource": 50,
      "zLoad": 50,
      "eSeries": 0
    }
  }),
});
console.log(response.status, await response.json());

package main

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

func main() {
	body := []byte(`{
	  "slug": "emi-filter-lc",
	  "inputs": {
	    "fc": 150,
	    "attenuationNeeded": 40,
	    "zSource": 50,
	    "zLoad": 50,
	    "eSeries": 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": "emi-filter-lc",
  "values": {
    "inductance": 53.05164769729845,
    "capacitance": 0.021220659078919377,
    "filterOrder": 2,
    "actualAttenuation": 40,
    "impedanceMatching": 50,
    "insertionLossBuilt": 53.89212170313197
  },
  "warnings": [],
  "errors": [],
  "provenance": {
    "method": "calculator:emi-filter-lc",
    "version": "api@3f2c1a9b04de",
    "formulaRef": "Williams & Taylor, \"Electronic Filter Design Handbook\" 4th ed.",
    "assumptions": [],
    "validRange": {
      "status": "inside",
      "bounds": {
        "fc": {
          "min": 0.1,
          "max": 100000,
          "unit": "kHz"
        },
        "attenuationNeeded": {
          "min": 6,
          "max": 120,
          "unit": "dB"
        },
        "zSource": {
          "min": 0.1,
          "max": 10000,
          "unit": "Ω"
        },
        "zLoad": {
          "min": 0.1,
          "max": 10000,
          "unit": "Ω"
        },
        "eSeries": {
          "min": 0,
          "max": 96,
          "unit": ""
        }
      },
      "outside": [],
      "model": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "fc": 150,
      "attenuationNeeded": 40,
      "zSource": 50,
      "zLoad": 50,
      "eSeries": 0
    },
    "seed": null,
    "elapsedSeconds": 0.003
  }
}

{
  "detail": [
    {
      "type": "float_parsing",
      "loc": [
        "body",
        "inputs",
        "fc"
      ],
      "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 pageLC EMI Filter Design Calculator, at these values