Documentation sections

RC Time Constant Calculator

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

Slug rc-time-constant. Calculate RC circuit time constant τ, charge time to 63.2% and 99%, and −3dB cutoff frequency. Essential for filter and timing circuit design.

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 rc-time-constant
Name	Unit	Range	Default	Description
resistancenumber	Ω	1 … 1000000000	10000	Resistance
capacitancenumber	nF	0.001 … 1000000000	100	Capacitance
rTolerancenumber	%	0 … 50	1	The resistor's tolerance, ±: 1 % (IEC 60062 code F) for a metal-film part, 5 % (J) for a general one
cTolerancenumber	%	0 … 80	5	The capacitor's tolerance, ±: 5 % (IEC 60062 code J) for C0G or film, 10 % (K) for X7R, 20 % (M) for an electrolytic
tauBudgetnumber	%	0.1 … 100	10	How far τ may stray from its nominal value, ±, as the circuit needs it

Outputs

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

Outputs of rc-time-constant
Field	Unit	Description
tauUsheadline	μs	Time Constant τ
t63us	μs	Time to 63.2%
t99us	μs	Time to 99%
f3db	Hz	−3dB Frequency
tauMinUs	μs	τ with both parts at the low end of their tolerance
tauMaxUs	μs	τ with both parts at the high end of their tolerance
tauErrorPct	%	The largest deviation of τ from nominal at the parts' tolerances: (1 + δR)(1 + δC) − 1, the high corner

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": "rc-time-constant",
    "inputs": {
      "resistance": 10000,
      "capacitance": 100,
      "rTolerance": 1,
      "cTolerance": 5,
      "tauBudget": 10
    }
  }'

import rftools

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

result = client.calculate("rc-time-constant", {
    "resistance": 10000,
    "capacitance": 100,
    "rTolerance": 1,
    "cTolerance": 5,
    "tauBudget": 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": "rc-time-constant",
    "inputs": {
      "resistance": 10000,
      "capacitance": 100,
      "rTolerance": 1,
      "cTolerance": 5,
      "tauBudget": 10
    }
  }),
});
console.log(response.status, await response.json());

package main

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

func main() {
	body := []byte(`{
	  "slug": "rc-time-constant",
	  "inputs": {
	    "resistance": 10000,
	    "capacitance": 100,
	    "rTolerance": 1,
	    "cTolerance": 5,
	    "tauBudget": 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": "rc-time-constant",
  "values": {
    "tauUs": 1000,
    "t63us": 1000,
    "t99us": 4605.170185988091,
    "f3db": 159.15494309189532,
    "tauMinUs": 940.5,
    "tauMaxUs": 1060.5,
    "tauErrorPct": 6.05
  },
  "warnings": [],
  "errors": [],
  "provenance": {
    "method": "calculator:rc-time-constant",
    "version": "api@3f2c1a9b04de",
    "formulaRef": null,
    "assumptions": [],
    "validRange": {
      "status": "inside",
      "bounds": {
        "resistance": {
          "min": 1,
          "max": 1000000000,
          "unit": "Ω"
        },
        "capacitance": {
          "min": 0.001,
          "max": 1000000000,
          "unit": "nF"
        },
        "rTolerance": {
          "min": 0,
          "max": 50,
          "unit": "%"
        },
        "cTolerance": {
          "min": 0,
          "max": 80,
          "unit": "%"
        },
        "tauBudget": {
          "min": 0.1,
          "max": 100,
          "unit": "%"
        }
      },
      "outside": [],
      "model": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "resistance": 10000,
      "capacitance": 100,
      "rTolerance": 1,
      "cTolerance": 5,
      "tauBudget": 10
    },
    "seed": null,
    "elapsedSeconds": 0.003
  }
}

{
  "detail": [
    {
      "type": "float_parsing",
      "loc": [
        "body",
        "inputs",
        "resistance"
      ],
      "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 pageRC Time Constant Calculator, at these values