Documentation sections

LC Filter Calculator — Low-Pass, High-Pass & Band-Pass

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

Slug filter-designer. Design passive Butterworth and Chebyshev LC ladder filters up to order 10. Calculate component values for low-pass, high-pass, and band-pass topologies. 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 filter-designer
Name	Unit	Range	Default	Description
responseTypenumber	none	0 … 1	0	Butterworth is maximally flat; Chebyshev Type I trades 0.5 dB of passband ripple for a steeper rolloffOne of: 0 Butterworth · 1 Chebyshev (0.5 dB ripple)
filterTypenumber	none	0 … 2	0	Filter TypeOne of: 0 Low-pass · 1 High-pass · 2 Band-pass
fcnumber	Hz	1 … 1000000000	1000	The design frequency: the −3 dB point of a Butterworth, the edge of the 0.5 dB ripple band of a Chebyshev, the centre of a band-pass
ordernumber	none	1 … 10	2	Filter order (1–10). Higher orders give steeper rolloff: n × 20 dB/decade.
impedancenumber	Ω	1 … 10000	50	Source/load impedance. 50 Ω is standard for RF systems; 600 Ω for audio.
qFactornumber	none	0.1 … 100	1	Higher Q = narrower bandwidth. Used only by the band-pass filter type.
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 filter-designer
Field	Unit	Description
C1_nFheadline	nF	First element, a capacitor: shunt in low-pass, series in high-pass, the shunt resonator's C in band-pass
L1_uH	μH	Second element (order ≥ 2), an inductor: series in low-pass, shunt in high-pass, the series resonator's L in band-pass
C3_nF	nF	Third element (order ≥ 3), a capacitor: shunt in low-pass, series in high-pass, the shunt resonator's C in band-pass
L4_uH	μH	Fourth element (order ≥ 4), an inductor: series in low-pass, shunt in high-pass, the series resonator's L in band-pass
C5_nF	nF	Fifth element (order ≥ 5), a capacitor: shunt in low-pass, series in high-pass, the shunt resonator's C in band-pass
L6_uH	μH	Sixth element (order ≥ 6), an inductor: series in low-pass, shunt in high-pass, the series resonator's L in band-pass
C7_nF	nF	Seventh element (order ≥ 7), a capacitor: shunt in low-pass, series in high-pass, the shunt resonator's C in band-pass
L8_uH	μH	Eighth element (order ≥ 8), an inductor: series in low-pass, shunt in high-pass, the series resonator's L in band-pass
C9_nF	nF	Ninth element (order ≥ 9), a capacitor: shunt in low-pass, series in high-pass, the shunt resonator's C in band-pass
L10_uH	μH	Tenth element (order = 10), an inductor: series in low-pass, shunt in high-pass, the series resonator's L in band-pass
bpL1_uH	μH	Band-pass only: the inductor in parallel with C₁ in element 1's shunt resonator, tuned to f_c
bpC2_nF	nF	Band-pass only: the capacitor in series with L₂ in element 2's series resonator, tuned to f_c
bpL3_uH	μH	Band-pass only: the inductor in parallel with C₃ in element 3's shunt resonator, tuned to f_c
bpC4_nF	nF	Band-pass only: the capacitor in series with L₄ in element 4's series resonator, tuned to f_c
bpL5_uH	μH	Band-pass only: the inductor in parallel with C₅ in element 5's shunt resonator, tuned to f_c
bpC6_nF	nF	Band-pass only: the capacitor in series with L₆ in element 6's series resonator, tuned to f_c
bpL7_uH	μH	Band-pass only: the inductor in parallel with C₇ in element 7's shunt resonator, tuned to f_c
bpC8_nF	nF	Band-pass only: the capacitor in series with L₈ in element 8's series resonator, tuned to f_c
bpL9_uH	μH	Band-pass only: the inductor in parallel with C₉ in element 9's shunt resonator, tuned to f_c
bpC10_nF	nF	Band-pass only: the capacitor in series with L₁₀ in element 10's series resonator, tuned to f_c
tau_ns	ns	RC time constant (1st-order Butterworth only)
attenuation_at_2fc	dB	Stopband attenuation at twice the cutoff frequency
bandwidth_Hz	Hz	Band-pass filter 3dB bandwidth (band-pass mode only)
fLow_Hz	Hz	Lower −3 dB corner frequency (band-pass mode only)
fHigh_Hz	Hz	Upper −3 dB corner frequency (band-pass mode only)
maxComponentError	%	Largest deviation from ideal value after E-series snapping
fcShiftPercent	%	How far rounding the parts moved the built ladder's −3 dB frequency (a band-pass: the centre of its −3 dB band) from the ideal ladder's
loadImpedance	Ω	The load the ladder is designed for: Z₀, except an even-order Chebyshev, which needs Z₀/g_{N+1} (g_{N+1}·Z₀ for high-pass)
cutoffBuilt_Hz	Hz	The −3 dB frequency of the ladder these part values build, between Z₀ and its load (low- and high-pass; 0 for band-pass)
fLowBuilt_Hz	Hz	Band-pass only: the lower −3 dB frequency of the ladder these part values build
fHighBuilt_Hz	Hz	Band-pass only: the upper −3 dB frequency of the ladder these part values build
passbandLoss_dB	dB	The largest loss of the built ladder in its passband: up to 0.8·f_c (low-pass), above 1.25·f_c (high-pass), or across the middle 80 % of the band (band-pass)
stopbandBuilt_dB	dB	The built ladder's loss where the prototype reaches twice its cutoff: at 2·f_c (low-pass), f_c/2 (high-pass) or the matching point above a band-pass's centre. Equal to the attenuation at 2×fc with ideal parts

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": "filter-designer",
    "inputs": {
      "responseType": 0,
      "filterType": 0,
      "fc": 1000,
      "order": 2,
      "impedance": 50,
      "qFactor": 1,
      "eSeries": 0
    }
  }'

import rftools

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

result = client.calculate("filter-designer", {
    "responseType": 0,
    "filterType": 0,
    "fc": 1000,
    "order": 2,
    "impedance": 50,
    "qFactor": 1,
    "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": "filter-designer",
    "inputs": {
      "responseType": 0,
      "filterType": 0,
      "fc": 1000,
      "order": 2,
      "impedance": 50,
      "qFactor": 1,
      "eSeries": 0
    }
  }),
});
console.log(response.status, await response.json());

package main

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

func main() {
	body := []byte(`{
	  "slug": "filter-designer",
	  "inputs": {
	    "responseType": 0,
	    "filterType": 0,
	    "fc": 1000,
	    "order": 2,
	    "impedance": 50,
	    "qFactor": 1,
	    "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": "filter-designer",
  "values": {
    "C1_nF": 4501.581580785531,
    "L1_uH": 11253.953951963827,
    "C3_nF": 0,
    "L4_uH": 0,
    "C5_nF": 0,
    "L6_uH": 0,
    "C7_nF": 0,
    "L8_uH": 0,
    "C9_nF": 0,
    "L10_uH": 0,
    "bpL1_uH": 0,
    "bpC2_nF": 0,
    "bpL3_uH": 0,
    "bpC4_nF": 0,
    "bpL5_uH": 0,
    "bpC6_nF": 0,
    "bpL7_uH": 0,
    "bpC8_nF": 0,
    "bpL9_uH": 0,
    "bpC10_nF": 0,
    "tau_ns": 0,
    "attenuation_at_2fc": 12.30448921378274,
    "bandwidth_Hz": 0,
    "fLow_Hz": 0,
    "fHigh_Hz": 0,
    "maxComponentError": 0,
    "fcShiftPercent": 0,
    "loadImpedance": 50,
    "cutoffBuilt_Hz": 1000.0000000000002,
    "fLowBuilt_Hz": 0,
    "fHighBuilt_Hz": 0,
    "passbandLoss_dB": 1.4909589106797274,
    "stopbandBuilt_dB": 12.30448921378274
  },
  "warnings": [],
  "errors": [],
  "provenance": {
    "method": "calculator:filter-designer",
    "version": "api@3f2c1a9b04de",
    "formulaRef": "Williams & Taylor, Electronic Filter Design Handbook 4th ed.; Zverev, Handbook of Filter Synthesis",
    "assumptions": [],
    "validRange": {
      "status": "inside",
      "bounds": {
        "responseType": {
          "min": 0,
          "max": 1,
          "unit": ""
        },
        "filterType": {
          "min": 0,
          "max": 2,
          "unit": ""
        },
        "fc": {
          "min": 1,
          "max": 1000000000,
          "unit": "Hz"
        },
        "order": {
          "min": 1,
          "max": 10,
          "unit": ""
        },
        "impedance": {
          "min": 1,
          "max": 10000,
          "unit": "Ω"
        },
        "qFactor": {
          "min": 0.1,
          "max": 100,
          "unit": ""
        },
        "eSeries": {
          "min": 0,
          "max": 96,
          "unit": ""
        }
      },
      "outside": [],
      "model": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "responseType": 0,
      "filterType": 0,
      "fc": 1000,
      "order": 2,
      "impedance": 50,
      "qFactor": 1,
      "eSeries": 0
    },
    "seed": null,
    "elapsedSeconds": 0.003
  }
}

{
  "detail": [
    {
      "type": "float_parsing",
      "loc": [
        "body",
        "inputs",
        "responseType"
      ],
      "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 Filter Calculator — Low-Pass, High-Pass & Band-Pass, at these values