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FIT (Flexible and Interoperable Data Transfer) is a binary data format developed by Garmin for storing activity, health, and device data from fitness devices. Every Garmin watch, cycling computer, and fitness tracker speaks FIT. When you finish a run, ride, or swim, the device writes a .fit file containing a timestamped stream of GPS coordinates, heart rate, cadence, power, temperature, and dozens of other metrics.
The format uses a compact binary encoding designed for resource-constrained embedded devices — smartwatches with limited storage and processing power. Data is organised into records conforming to predefined message types: session, lap, record (per-second data points), event, device_info, and many more. The FIT SDK defines hundreds of fields across these message types, and manufacturers can add custom developer fields.
FIT files are the lingua franca of the fitness tech ecosystem. Garmin Connect, Strava, TrainingPeaks, Zwift, and Wahoo all import and export FIT. The format carries more data than GPX (which is limited to GPS tracks and basic extensions) and is more compact. If you're analysing training data programmatically, the FIT SDK provides parsers for Python, Java, C, and C++.