How to Merge FIT Files from Garmin, Wahoo & Coros

What Is a FIT File?

FIT (Flexible and Interoperable Data Transfer) is the binary file format used by Garmin, Wahoo, Coros, Suunto, and most modern cycling computers. Unlike GPX, which is human-readable XML, FIT is a compact binary format designed for efficiency.

FIT files store richer data than GPX — including power meter metrics, training effect scores, device-specific fields, and sensor battery status. When you export from Garmin Connect or download directly from a Wahoo head unit, you're almost always working with FIT files.

The downside? FIT files are harder to work with. You can't open them in a text editor, and most online tools only handle GPX. That's where TrailBlender comes in.

Why Merge FIT Files Instead of Converting to GPX First?

A common workaround is to convert FIT to GPX, merge the GPX files, and convert back. This is a bad idea for three reasons:

1. Data loss — FIT stores dozens of fields that GPX doesn't support: Training Effect, performance condition, power balance, pedal smoothness, and more. Converting to GPX strips all of this.
2. Precision loss — FIT stores coordinates as semicircles (32-bit integers), which is more precise than GPX's decimal degrees. Round-tripping through GPX introduces tiny coordinate errors.
3. Extra steps — Converting to GPX and back adds two unnecessary steps and two opportunities for errors.

TrailBlender merges FIT files directly, preserving every data field from every device.

The Hard Way: FIT SDK and Command-Line Tools

Garmin publishes a FIT SDK with command-line tools for parsing and creating FIT files. You could theoretically:

1. Decode both FIT files to CSV using the FIT SDK
2. Merge the CSV data manually, aligning timestamps
3. Re-encode the merged CSV back to FIT

This requires installing Java, understanding the FIT protocol's message/field structure, and writing custom scripts to handle timestamp alignment and data channel conflicts. It's doable if you're a developer with time to spare. For everyone else, it's not practical.

The Easy Way: Merge with TrailBlender

TrailBlender reads and writes FIT files natively — no conversion needed.

1. Get your FIT files — Download from Garmin Connect, copy from your Wahoo via USB, or pull from whatever device recorded the activity.
2. Open TrailBlender and drag in all your FIT files.
3. TrailBlender reads the binary data — It parses every FIT message type: records, laps, sessions, device info, and sensor data.
4. Choose your data sources — Select which device provides GPS, power, heart rate, cadence, and other channels.
5. Preview the merge — Check the map and data graphs to verify everything looks correct.
6. Export as FIT — The merged file retains all FIT-specific fields. Upload to Garmin Connect, Strava, TrainingPeaks, or any platform that accepts FIT.

FIT vs GPX: When to Use Each Format

• Use FIT when you need maximum data fidelity — uploading to Garmin Connect, TrainingPeaks, or any platform that preserves advanced metrics.
• Use GPX when you need maximum compatibility — sharing routes with others, uploading to platforms that don't support FIT, or archiving in a human-readable format.
• TrailBlender can export both — Merge your files and export in whichever format your destination platform prefers. You can even export the same merge as both FIT and GPX for different uses.