FIT File Decoder

What this tool does

A .fit file is the compact binary format Garmin, Wahoo, and other ANT devices write for a recorded activity. This decoder reads that file and turns it into something you can actually read:

• Summary — protocol/profile version, how many record messages were decoded, the time range, a GPS bounding box, and the session totals and averages (sport, distance, moving/elapsed time, calories, speed, heart rate, power, ascent/descent).
• CSV — one row per sampled record with timestamp,latitude,longitude,altitude_m,distance_m,speed_mps,heart_rate,cadence,power, ready for a spreadsheet.
• GPX — a GPX 1.1 <trk> with <ele> elevation, <time> stamps, and Garmin TrackPointExtension heart-rate/cadence plus <power> extensions, for any GPX-aware map or training app.

Input shape and limits

The tool works on text, so paste the file as base64, not the raw binary. Convert your .fit first — for example base64 ride.fit on macOS/Linux, or certutil -encode ride.fit out.txt on Windows — then paste the result. Both the standard (+//) and URL-safe (-/_) alphabets are accepted, = padding is optional, and whitespace or line breaks in the pasted text are ignored.

• The decoded bytes must begin with a valid FIT header carrying the .FIT signature; anything else is rejected as "not a FIT file".
• Maximum decoded size is 8 MiB — enough for very long activities.
• Only record (per-second samples) and session (activity totals) messages are surfaced. Positions are stored as semicircles and converted to degrees; the FIT "invalid" sentinels (all-0xFF, or 0x7FFFFFFF for coordinates) drop the field rather than printing a garbage value.
• Little- and big-endian files, compressed-timestamp records, and developer fields are all handled; developer field bytes are skipped, and unknown message types are ignored instead of erroring.

Worked example

Decode a short cycling activity (three GPS records plus a session summary) with format=summary and you get:

FIT file summary
================
Protocol version: 1.0
Profile version:  21.40
Data section:     192 bytes
Records decoded:  3
Time range:       2024-03-09T08:00:00Z → 2024-03-09T08:02:00Z (2:00)
GPS points:       3 (lat 40.000000..40.002000, lon -105.001000..-105.000000)

Session
-------
Sport:            cycling
Start:            2024-03-09T08:00:00Z
Total distance:   0.25 km
Total time:       2:00 (elapsed 2:00)
Calories:         42 kcal
Speed:            avg 21.6  max 25.2 (km/h)
Heart rate:       avg 137  max 150 bpm
Power:            avg 200  max 250 W

Switch to format=csv and the same file becomes a table whose first data row is:

2024-03-09T08:00:00Z,40.0000000,-105.0000000,1600.0,0.00,5.000,120,80,150

Switch to format=gpx for a <trk> of three <trkpt> elements, each carrying <ele>, <time>, <gpxtpx:hr>, <gpxtpx:cad>, and <power>.

Limits and edge cases

• A record only carries whatever the device sampled at that instant, so any column can be blank — a bike computer without GPS produces CSV rows with empty latitude/longitude, and GPX skips points that have no position at all.
• Speed and altitude prefer the "enhanced" 32-bit fields when a file provides them, falling back to the classic 16-bit values otherwise.
• The summary reads the first session message. Multi-sport files show that first session's sport and totals.
• This is a read-only decoder: it never re-encodes or edits the FIT file, and laps, HR zones, and device-info messages are outside its scope.

FAQ