Documentation sections

Johnson-Nyquist Thermal Noise Calculator

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

Slug johnson-noise. Calculate thermal noise voltage, power, and spectral density for any resistor. Determine Johnson-Nyquist noise floor for low-noise circuit design. 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 johnson-noise
Name	Unit	Range	Default	Description
resistancenumber	Ω	at least 0.001	50	Resistance
temperaturenumber	°C	at least −273	25	Physical temperature of the resistor
bandwidthnumber	MHz	at least 0.000001	1	Measurement or system bandwidth

Outputs

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

Outputs of johnson-noise
Field	Unit	Description
noiseVoltageheadline	nV	RMS Noise Voltage
noisePower	dBm	Available Noise Power
noisePowerDensity	dBm/Hz	kT in dBm/Hz (−174 dBm/Hz at 290 K)
noiseCurrentDensity	pA/√Hz	Noise Current Density

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": "johnson-noise",
    "inputs": {
      "resistance": 50,
      "temperature": 25,
      "bandwidth": 1
    }
  }'

import rftools

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

result = client.calculate("johnson-noise", {"resistance": 50, "temperature": 25, "bandwidth": 1})
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": "johnson-noise",
    "inputs": {
      "resistance": 50,
      "temperature": 25,
      "bandwidth": 1
    }
  }),
});
console.log(response.status, await response.json());

package main

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

func main() {
	body := []byte(`{
	  "slug": "johnson-noise",
	  "inputs": {
	    "resistance": 50,
	    "temperature": 25,
	    "bandwidth": 1
	  }
	}`)
	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": "johnson-noise",
  "values": {
    "noiseVoltage": 907.3483337175421,
    "noisePower": -113.85481903640007,
    "noisePowerDensity": -173.8548190364001,
    "noiseCurrentDensity": 18.146966674350843
  },
  "warnings": [],
  "errors": [],
  "provenance": {
    "method": "calculator:johnson-noise",
    "version": "api@3f2c1a9b04de",
    "formulaRef": null,
    "assumptions": [],
    "validRange": {
      "status": "inside",
      "bounds": {
        "resistance": {
          "min": 0.001,
          "max": null,
          "unit": "Ω"
        },
        "temperature": {
          "min": -273,
          "max": null,
          "unit": "°C"
        },
        "bandwidth": {
          "min": 0.000001,
          "max": null,
          "unit": "MHz"
        }
      },
      "outside": [],
      "model": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "resistance": 50,
      "temperature": 25,
      "bandwidth": 1
    },
    "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 pageJohnson-Nyquist Thermal Noise Calculator, at these values