Documentation sections

VSWR & Return Loss Calculator

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

Slug vswr-return-loss. Convert between VSWR, return loss, reflection coefficient, and mismatch loss instantly. Get reflected and transmitted power percentages. 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 vswr-return-loss
Name	Unit	Range	Default	Description
vswrnumber	:1	1 … 100	1.5	Voltage Standing Wave Ratio (1:1 = perfect match)

Outputs

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

Outputs of vswr-return-loss
Field	Unit	Description
returnLossheadline	dB	Return Loss
gamma	none	Reflection Coefficient
mismatchLoss	dB	Mismatch Loss
reflectedPct	%	Reflected Power
transmittedPct	%	Transmitted Power

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": "vswr-return-loss",
    "inputs": {
      "vswr": 1.5
    }
  }'

import rftools

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

result = client.calculate("vswr-return-loss", {"vswr": 1.5})
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": "vswr-return-loss",
    "inputs": {
      "vswr": 1.5
    }
  }),
});
console.log(response.status, await response.json());

package main

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

func main() {
	body := []byte(`{
	  "slug": "vswr-return-loss",
	  "inputs": {
	    "vswr": 1.5
	  }
	}`)
	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": "vswr-return-loss",
  "values": {
    "gamma": 0.2,
    "returnLoss": 13.979400086720375,
    "mismatchLoss": 0.17728766960431602,
    "reflectedPct": 4.000000000000001,
    "transmittedPct": 96
  },
  "warnings": [],
  "errors": [],
  "provenance": {
    "method": "calculator:vswr-return-loss",
    "version": "api@3f2c1a9b04de",
    "formulaRef": "Pozar, \"Microwave Engineering\" 4th ed., Chapter 2",
    "assumptions": [],
    "validRange": {
      "status": "inside",
      "bounds": {
        "vswr": {
          "min": 1,
          "max": 100,
          "unit": ":1"
        }
      },
      "outside": [],
      "model": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "vswr": 1.5
    },
    "seed": null,
    "elapsedSeconds": 0.003
  }
}

{
  "detail": [
    {
      "type": "float_parsing",
      "loc": [
        "body",
        "inputs",
        "vswr"
      ],
      "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 pageVSWR & Return Loss Calculator, at these values