Documentation sections

Free-Space Path Loss Calculator

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

Slug free-space-path-loss. Calculate free-space path loss (FSPL) in dB using the Friis equation. Enter frequency and distance for wireless link budget analysis. 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 free-space-path-loss
Name	Unit	Range	Default	Description
frequencynumber	MHz	at least 0.1	2400	Signal frequency in MHz
distancenumber	km	at least 0.001	1	Distance between transmitter and receiver in km

Outputs

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

Outputs of free-space-path-loss
Field	Unit	Description
fspl_dBheadline	dB	Free-Space Path Loss
fspl_linear	none	FSPL (linear ratio)

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": "free-space-path-loss",
    "inputs": {
      "frequency": 2400,
      "distance": 1
    }
  }'

import rftools

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

result = client.calculate("free-space-path-loss", {"frequency": 2400, "distance": 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": "free-space-path-loss",
    "inputs": {
      "frequency": 2400,
      "distance": 1
    }
  }),
});
console.log(response.status, await response.json());

package main

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

func main() {
	body := []byte(`{
	  "slug": "free-space-path-loss",
	  "inputs": {
	    "frequency": 2400,
	    "distance": 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": "free-space-path-loss",
  "values": {
    "fspl_dB": 100.0520080561155,
    "fspl_linear": 10120472884.315342
  },
  "warnings": [],
  "errors": [],
  "provenance": {
    "method": "calculator:free-space-path-loss",
    "version": "api@3f2c1a9b04de",
    "formulaRef": "ITU-R P.525-4, Calculation of free-space attenuation",
    "assumptions": [],
    "validRange": {
      "status": "inside",
      "bounds": {
        "frequency": {
          "min": 0.1,
          "max": null,
          "unit": "MHz"
        },
        "distance": {
          "min": 0.001,
          "max": null,
          "unit": "km"
        }
      },
      "outside": [],
      "model": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "frequency": 2400,
      "distance": 1
    },
    "seed": null,
    "elapsedSeconds": 0.003
  }
}

{
  "detail": [
    {
      "type": "float_parsing",
      "loc": [
        "body",
        "inputs",
        "frequency"
      ],
      "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 pageFree-Space Path Loss Calculator, at these values