FIT Format

The @kaiord/fit package provides reading and writing of Garmin FIT workout files using the official Garmin FIT SDK.

Installation

pnpm add @kaiord/core @kaiord/fit

Usage

Pre-built adapters

import { fromBinary, toBinary } from "@kaiord/core";
import { fitReader, fitWriter } from "@kaiord/fit";
import { readFile, writeFile } from "node:fs/promises";

// FIT to KRD
const buffer = await readFile("workout.fit");
const krd = await fromBinary(new Uint8Array(buffer), fitReader);

// KRD to FIT
const fitBuffer = await toBinary(krd, fitWriter);
await writeFile("output.fit", fitBuffer);

Factory with custom logger

import { createConsoleLogger } from "@kaiord/core";
import { createFitReader, createFitWriter } from "@kaiord/fit";

const logger = createConsoleLogger();
const reader = createFitReader(logger);
const writer = createFitWriter(logger);

API

Function	Description
fitReader	Pre-built FIT binary reader
fitWriter	Pre-built FIT binary writer
createFitReader(logger?)	Factory for FIT reader with optional logger
createFitWriter(logger?)	Factory for FIT writer with optional logger

Supported features

• Workout files (structured workout steps)
• Activity files (recorded activity data)
• Course files (GPS routes)
• Lap messages
• Record messages (time-series data)
• Event messages
• Session and activity metadata
• Developer fields (preserved in extensions.fit)
• Unknown messages (preserved in extensions.fit)

FIT-specific notes

FIT is Garmin's binary protocol. When converting FIT to KRD:

• Duration values are converted from milliseconds to seconds
• Power targets with Garmin's +1000 offset are normalized
• Sub-sport values are mapped from camelCase to snake_case
• Developer fields and unknown messages are preserved in extensions.fit

Ready to convert? Follow the Quick Start.