Documentation sections

RF Power Density Calculator

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

Slug power-density. Calculate power density in W/m² from EIRP and distance using S = EIRP/(4πd²). Get electric field E in V/m, magnetic field H in A/m, and check FCC & ICNIRP EMF safety exposure limits instantly.

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 power-density
Name	Unit	Range	Default	Description
eirpnumber	dBm	−30 … 80	30	Equivalent Isotropically Radiated Power (transmitter power + antenna gain)
distancenumber	m	at least 0.01	10	Distance from the antenna to observation point (far field assumed)
frequencynumber	GHz	at least 0.001	2.4	Sets the exposure limit; the far-field power density itself does not depend on it
standardnumber	none	0 … 1	0	The exposure limits the power density is checked againstOne of: 0 FCC 47 CFR §1.1310 · 1 ICNIRP 2020
exposureClassnumber	none	0 … 1	0	General public (uncontrolled) or occupational (controlled) exposureOne of: 0 General public · 1 Occupational
groundReflectionnumber	none	0 … 1	0	FCC OET Bulletin 65: a ground reflection adding in phase raises the power density by 1.6² = 2.56One of: 0 Free space · 1 Ground reflection ×2.56

Outputs

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

Outputs of power-density
Field	Unit	Description
powerDensityheadline	mW/m²	RF power density at given distance: S = EIRP / (4π·d²)
efield	V/m	Far-field electric field strength: E = √(S × η₀), η₀ = µ₀·c ≈ 376.73 Ω
hfield	mA/m	Far-field magnetic field strength: H = E / η₀
planeWaveImpedance	Ω	Free-space plane-wave impedance η₀ = µ₀·c ≈ 376.73 Ω
limitDensity	mW/m²	The power-density limit of the chosen standard and exposure class at the frequency
limitPercent	%	The power density as a percentage of the limit; above 100 % does not comply
minDistance	m	The distance at which the power density equals the limit; at or beyond it the exposure complies
maxEirp	dBm	The EIRP at which the power density at this distance equals the limit; at or below it the exposure complies

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": "power-density",
    "inputs": {
      "eirp": 30,
      "distance": 10,
      "frequency": 2.4,
      "standard": 0,
      "exposureClass": 0,
      "groundReflection": 0
    }
  }'

import rftools

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

result = client.calculate("power-density", {
    "eirp": 30,
    "distance": 10,
    "frequency": 2.4,
    "standard": 0,
    "exposureClass": 0,
    "groundReflection": 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": "power-density",
    "inputs": {
      "eirp": 30,
      "distance": 10,
      "frequency": 2.4,
      "standard": 0,
      "exposureClass": 0,
      "groundReflection": 0
    }
  }),
});
console.log(response.status, await response.json());

package main

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

func main() {
	body := []byte(`{
	  "slug": "power-density",
	  "inputs": {
	    "eirp": 30,
	    "distance": 10,
	    "frequency": 2.4,
	    "standard": 0,
	    "exposureClass": 0,
	    "groundReflection": 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": "power-density",
  "values": {
    "powerDensity": 0.7957747154594766,
    "efield": 0.547533065817216,
    "hfield": 1.453382024100681,
    "planeWaveImpedance": 376.7303136668535,
    "limitDensity": 10000,
    "limitPercent": 0.007957747154594767,
    "minDistance": 0.08920620580763855,
    "maxEirp": 70.99209864022097
  },
  "warnings": [],
  "errors": [],
  "provenance": {
    "method": "calculator:power-density",
    "version": "api@3f2c1a9b04de",
    "formulaRef": "ITU-R P.525-4, Calculation of free-space attenuation — power flux density",
    "assumptions": [],
    "validRange": {
      "status": "inside",
      "bounds": {
        "eirp": {
          "min": -30,
          "max": 80,
          "unit": "dBm"
        },
        "distance": {
          "min": 0.01,
          "max": null,
          "unit": "m"
        },
        "frequency": {
          "min": 0.001,
          "max": null,
          "unit": "GHz"
        },
        "standard": {
          "min": 0,
          "max": 1,
          "unit": ""
        },
        "exposureClass": {
          "min": 0,
          "max": 1,
          "unit": ""
        },
        "groundReflection": {
          "min": 0,
          "max": 1,
          "unit": ""
        }
      },
      "outside": [],
      "model": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "eirp": 30,
      "distance": 10,
      "frequency": 2.4,
      "standard": 0,
      "exposureClass": 0,
      "groundReflection": 0
    },
    "seed": null,
    "elapsedSeconds": 0.003
  }
}

{
  "detail": [
    {
      "type": "float_parsing",
      "loc": [
        "body",
        "inputs",
        "eirp"
      ],
      "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 pageRF Power Density Calculator, at these values