Documentation sections

Decoupling Capacitor Selection Calculator

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

Slug decoupling-capacitor. Calculate decoupling capacitor SRF, impedance at target frequency, and number of caps needed for power integrity. Includes ESR/ESL modeling. 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 decoupling-capacitor
Name	Unit	Range	Default	Description
capacitancenumber	nF	at least 0.001	100	Capacitance
esrnumber	mΩ	at least 0.1	50	Equivalent series resistance of the capacitor
eslnumber	nH	at least 0.01	1	Equivalent series inductance (package parasitic)
targetFreqnumber	MHz	at least 0.001	100	Frequency at which decoupling impedance is evaluated
supplyImpedancenumber	mΩ	at least 0.001	10	Maximum allowable PDN impedance at the target frequency

Outputs

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

Outputs of decoupling-capacitor
Field	Unit	Description
selfResonantFreq	MHz	Self-Resonant Frequency
impedanceAtSrf	mΩ	Impedance at SRF
impedanceAtTarget	mΩ	Impedance at Target Freq
effectiveRange	MHz	Effective Bypass Range (upper)
numCapacitorsheadline	none	Capacitors Needed

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": "decoupling-capacitor",
    "inputs": {
      "capacitance": 100,
      "esr": 50,
      "esl": 1,
      "targetFreq": 100,
      "supplyImpedance": 10
    }
  }'

import rftools

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

result = client.calculate("decoupling-capacitor", {
    "capacitance": 100,
    "esr": 50,
    "esl": 1,
    "targetFreq": 100,
    "supplyImpedance": 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": "decoupling-capacitor",
    "inputs": {
      "capacitance": 100,
      "esr": 50,
      "esl": 1,
      "targetFreq": 100,
      "supplyImpedance": 10
    }
  }),
});
console.log(response.status, await response.json());

package main

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

func main() {
	body := []byte(`{
	  "slug": "decoupling-capacitor",
	  "inputs": {
	    "capacitance": 100,
	    "esr": 50,
	    "esl": 1,
	    "targetFreq": 100,
	    "supplyImpedance": 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": "decoupling-capacitor",
  "values": {
    "selfResonantFreq": 15.915494309189533,
    "impedanceAtSrf": 50,
    "impedanceAtTarget": 614.4407855950648,
    "effectiveRange": 47.7464829275686,
    "numCapacitors": 62
  },
  "warnings": [],
  "errors": [],
  "provenance": {
    "method": "calculator:decoupling-capacitor",
    "version": "api@3f2c1a9b04de",
    "formulaRef": null,
    "assumptions": [],
    "validRange": {
      "status": "inside",
      "bounds": {
        "capacitance": {
          "min": 0.001,
          "max": null,
          "unit": "nF"
        },
        "esr": {
          "min": 0.1,
          "max": null,
          "unit": "mΩ"
        },
        "esl": {
          "min": 0.01,
          "max": null,
          "unit": "nH"
        },
        "targetFreq": {
          "min": 0.001,
          "max": null,
          "unit": "MHz"
        },
        "supplyImpedance": {
          "min": 0.001,
          "max": null,
          "unit": "mΩ"
        }
      },
      "outside": [],
      "model": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "capacitance": 100,
      "esr": 50,
      "esl": 1,
      "targetFreq": 100,
      "supplyImpedance": 10
    },
    "seed": null,
    "elapsedSeconds": 0.003
  }
}

{
  "detail": [
    {
      "type": "float_parsing",
      "loc": [
        "body",
        "inputs",
        "capacitance"
      ],
      "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 pageDecoupling Capacitor Selection Calculator, at these values