FIT File Deep Dive: Format, Editing, Repair & Merging Guide

2026-06-10 · floatinghotpot

The short answer: FIT (Flexible and Interoperable Data Transfer) is Garmin's binary sports data format — the de facto standard across virtually all sports devices and platforms. Understanding FIT structure gives you the "source code" of your sports data — view, repair, edit, and merge activities from any platform.

What Is a FIT File?

FIT is Garmin's open binary file format, released in 2005, designed specifically for storing and transferring sports and health data. It's used by Garmin, Strava, Zwift, COROS, and virtually every major sports platform and device.

A FIT file contains:

Activity Records: Second-by-second GPS coordinates, heart rate, power, cadence, speed, elevation.
Laps: Per-kilometer or per-mile split statistics.
Sessions: Whole-activity summaries — total distance, average pace, calories.
Device Info: Recording device product ID, firmware version, battery level.
User Profile: Gender, age, weight, FTP, and other personal metrics.

FIT is binary — you can't open it in a text editor. Use dedicated tools like Golden Cheetah (free and open source), Garmin's official FitSDK, or online FIT viewers.

Common FIT File Issues & Fixes

Issue 1: Device ID Incompatibility

Platforms have different recognition strategies for the device Product ID in FIT files. For example, MyWhoosh-generated FIT files aren't recognized as valid training data by Garmin Connect. Fix: rewrite the device Product ID to a Garmin-recognized ID. GarSync's FIT repair supports per-account auto-rewrite.

Issue 2: Coordinate System Mismatch

Some China-market devices (e.g., Magene bike computers) output GCJ-02 coordinates, while global platforms expect WGS-84. Direct sync causes 100-700m GPS offsets. Coordinate conversion is needed before upload — see our GCJ-02 correction guide.

Issue 3: Missing HR/Power Zones

Some devices or apps generate FIT files without heart rate and power zone distributions. GarSync auto-calculates and fills these fields from raw sensor data before upload.

Issue 4: Timestamp Offset

Some devices record FIT files with UTC timezone errors (commonly +8 hours), causing activities to show wrong start times on Strava. The fix is batch-adjusting timestamp fields.

Issue 5: Missing Cadence Data

If you don't have a dedicated cadence sensor but use a power meter, cadence data is often embedded in the power telemetry stream. GarSync auto-extracts and fills the cadence field.

Recommended FIT Tools

Golden Cheetah: Free open-source desktop training analysis software — opens FIT files directly with full data field visibility.
Garmin FitSDK: Garmin's official FIT processing toolkit — includes FIT→CSV conversion, CSV→FIT packing, and command-line utilities.
GPXSee: Lightweight GPS track viewer supporting FIT/GPX/TCX formats.
GarSync FIT Repair: One-tap in-app fixes — device ID rewrite, coordinate conversion, field completion.

FAQ

Q: What's the difference between FIT, GPX, and TCX?

A: GPX and TCX are XML text formats — human-readable but larger files. FIT is binary — smaller, faster to parse, and supports richer data types (running dynamics, cycling power, etc.). FIT is the industry standard; GPX/TCX are mainly used for route sharing.

Q: Can editing a FIT file corrupt my data?

A: Editing metadata fields (device ID, timestamps) won't affect core sensor data. But modifying Record-level sensor data (HR, power) will distort your data. GarSync's repair only touches metadata — sensor data stays intact.

Q: Can I write my own scripts to process FIT files?

A: Yes. Garmin's open-source FitSDK supports Python, Java, C#, and more. Use FitDecoder to read FIT files and FitEncoder to generate new ones.