---
title: "Calculate free-space path loss (FSPL) in decibels using the ITU-R P.525 standard formula. Given a frequency in MHz and distance in kilometres, returns the expected signal attenuation in an ideal line-of-sight radio link with no obstacles, reflections, or atmospheric absorption. Also computes the RF wavelength. Use this to estimate baseline path loss before adding fade margins, antenna gains, or environmental corrections. Feeds directly into link_budget for full end-to-end analysis."
method: POST
path: "/api/fspl"
tags: ["rf"]
---

# Calculate free-space path loss (FSPL) in decibels using the ITU-R P.525 standard formula. Given a frequency in MHz and distance in kilometres, returns the expected signal attenuation in an ideal line-of-sight radio link with no obstacles, reflections, or atmospheric absorption. Also computes the RF wavelength. Use this to estimate baseline path loss before adding fade margins, antenna gains, or environmental corrections. Feeds directly into link_budget for full end-to-end analysis.

`POST /api/fspl`

## Request

- Base URL: `https://calc.engineer`
- URL: `https://calc.engineer/api/fspl`
- Auth: none declared

## Request body

- object
  - `freq_mhz` number, required: Carrier frequency in megahertz (MHz)
  - `dist_km` number, required: Distance between transmitter and receiver in kilometres

## Response `200`

Computed result for fspl

- object
  - `result` object, required
    - `fspl_db` number, required: Free-space path loss in decibels (dB)
    - `wavelength_m` number, required: RF wavelength in metres
  - `tool` string, required
  - `version` string, required
  - `computed_at` string, date-time, required
  - `chain` object, required
    - `feeds` unknown[]
      - unknown
    - `accepts_from` unknown[]
      - unknown

## Other responses

- `400`: Validation error or invalid JSON
- `404`: Tool not found

---

[API](https://skmtc.dev/calc/apis/calc-engineer-api.md) · [All operations](https://skmtc.dev/calc/apis/calc-engineer-api/llms.txt) · [OpenAPI document](https://skmtc.dev/calc/apis/calc-engineer-api/revisions/8830480ec488?raw)