Documentation sections

Coaxial Cable Loss Calculator

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

Slug coax-loss. Calculate RF cable attenuation for LMR-400, RG-58, RG-213, and more. Enter cable type, frequency, and length to get insertion loss in dB. 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 coax-loss
Name	Unit	Range	Default	Description
cableIndexnumber	none	0 … 17	0	Attenuation is k₁√f + k₂f from the cable's datasheet: Times Microwave's own coefficients, or a fit to Belden's or CommScope's tableOne of: 0 LMR-400 · 1 LMR-240 · 2 LMR-200 · 3 LMR-100A · 4 RG-58/U (Belden 8240) · 5 RG-8X (Belden 9258) · 6 RG-213/U (Belden 8267) · 7 RG-174/U (Belden 8216) · 8 Belden 9913 · 9 Andrew LDF4-50A (1/2") · 10 LMR-600 · 11 LMR-195 · 12 LMR-400-UltraFlex · 13 LMR-400-LLPL (plenum) · 14 RG-316 (Belden 83284) · 15 RG-6 (Belden 1694A, 75 Ω) · 16 Andrew LDF5-50A (7/8") · 17 LMR-900
frequencyMHznumber	MHz	1 … 6000	435	Frequency
lengthMnumber	m	0.1 … 10000	10	Cable Length
inputPowernumber	dBm	−50 … 80	30	Power into the cable; the power out is this less the insertion loss, into a matched load

Outputs

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

Outputs of coax-loss
Field	Unit	Description
attenuation_dBheadline	dB	Insertion Loss
lossPer100m	dB/100m	Loss per 100 m
powerRemaining_pct	%	Power at Load
voltageRemaining_pct	%	Voltage at Load
powerOut_dBm	dBm	Input power less the insertion loss
powerOut_W	W	Output Power (linear)
lossPerMeter	dB/m	Loss per Metre

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": "coax-loss",
    "inputs": {
      "cableIndex": 0,
      "frequencyMHz": 435,
      "lengthM": 10,
      "inputPower": 30
    }
  }'

import rftools

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

result = client.calculate("coax-loss", {"cableIndex": 0, "frequencyMHz": 435, "lengthM": 10, "inputPower": 30})
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": "coax-loss",
    "inputs": {
      "cableIndex": 0,
      "frequencyMHz": 435,
      "lengthM": 10,
      "inputPower": 30
    }
  }),
});
console.log(response.status, await response.json());

package main

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

func main() {
	body := []byte(`{
	  "slug": "coax-loss",
	  "inputs": {
	    "cableIndex": 0,
	    "frequencyMHz": 435,
	    "lengthM": 10,
	    "inputPower": 30
	  }
	}`)
	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": "coax-loss",
  "values": {
    "attenuation_dB": 0.8739042554236128,
    "lossPer100m": 8.739042554236129,
    "powerRemaining_pct": 81.77293287183377,
    "voltageRemaining_pct": 90.42838761795645,
    "powerOut_dBm": 29.126095744576386,
    "powerOut_W": 0.8177293287183375,
    "lossPerMeter": 0.08739042554236129
  },
  "warnings": [],
  "errors": [],
  "provenance": {
    "method": "calculator:coax-loss",
    "version": "api@3f2c1a9b04de",
    "formulaRef": "Times Microwave LMR datasheets (k₁, k₂ at VSWR 1.0, +25 °C); Belden technical data sheets 8240, 9258, 8267, 8216, 9913, 83284, 1694A; CommScope/Andrew LDF4-50A and LDF5-50A specifications (20 °C)",
    "assumptions": [],
    "validRange": {
      "status": "inside",
      "bounds": {
        "cableIndex": {
          "min": 0,
          "max": 17,
          "unit": ""
        },
        "frequencyMHz": {
          "min": 1,
          "max": 6000,
          "unit": "MHz"
        },
        "lengthM": {
          "min": 0.1,
          "max": 10000,
          "unit": "m"
        },
        "inputPower": {
          "min": -50,
          "max": 80,
          "unit": "dBm"
        }
      },
      "outside": [],
      "model": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "cableIndex": 0,
      "frequencyMHz": 435,
      "lengthM": 10,
      "inputPower": 30
    },
    "seed": null,
    "elapsedSeconds": 0.003
  }
}

{
  "detail": [
    {
      "type": "float_parsing",
      "loc": [
        "body",
        "inputs",
        "cableIndex"
      ],
      "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 pageCoaxial Cable Loss Calculator, at these values