What is a FIT file? What your watch actually writes

A FIT file is the recording a watch or a bike computer makes, in the form the watch itself writes it. Garmin designed the format — *Flexible and Interoperable Data Transfer* — and Wahoo, Coros, Suunto, Polar and Zwift all write it too. When you export an activity from Garmin Connect, or take a bulk export from Strava, the file you get is usually a FIT.

It is binary, not text

Open a FIT file in a text editor and you get a screen of symbols. That is not corruption. The format packs each second's data into a few bytes instead of a line of text, which is why a two-hour ride is a few hundred kilobytes as FIT and several megabytes as GPX. The file describes its own layout as it goes — *definition messages* that say which fields follow and how wide they are — so a reader that only knows a few fields can still walk through the whole file.

What is inside

• Records, one per second: position, elevation, speed, distance so far, heart rate, cadence, power, temperature. Each only if the sensor was there.
• Laps and sessions: the watch's own totals. Distance, moving and elapsed time, ascent and descent, average and maximum for each sensor, calories, training effect. These are the numbers the watch showed you when you pressed stop.
• Events: start, pause, resume, the timer.
• The device: which watch, which firmware, which sensors were paired.

A GPX has only the first of those, and only part of it. That is the difference that matters: a FIT carries the watch's accounting, a GPX carries the trace. GPX, FIT or TCX goes through what each holds.

What opens one

• The FIT file viewer draws the route with distance, time and elevation in your browser, and reads `.fit.gz` as it is. Nothing installed, nothing uploaded.
• Garmin Connect and Strava by upload, as an activity.
• Training software — Golden Cheetah, TrainingPeaks, intervals.icu — for the power and heart-rate analysis the file was made for.
• Garmin's FIT SDK if you write code.

To turn one into a GPX for a tool that does not read FIT: FIT to GPX, in the browser.

.fit.gz

The `.gz` on the end means the file is compressed with gzip, which halves it again. Strava's bulk export delivers most activities that way. The viewer and the converter here unzip it on the fly; Strava itself, according to its own help pages, wants a FIT downloaded from Garmin Connect unzipped before it is uploaded. .fit.gz files is the short version.

When a FIT file goes wrong

A watch that lost power mid-ride, or a transfer that stopped early, leaves a file that Garmin Connect or Strava refuses. Sometimes it is repairable: repair a FIT file. A FIT with no positions at all is usually an indoor session, not a fault — the trainer recorded power and heart rate and there was no GPS to record.

For a print

The editor reads FIT directly, including the compressed kind, and uses the session's own totals for the numbers under the line — the same distance and time the watch showed, not a recalculation. What your file carries lists field by field what makes it onto paper.