The FIT file: what it holds, and when to use it
FIT is the format your watch actually records an activity in. The summary you look at is only an extract. Understanding what the file contains explains why some analyses are possible and others are not.
Last updated: 2026-08-24
Summary versus file
An activity summary is a few dozen values: duration, distance, average pace, average heart rate. That is what interfaces display and what most programming interfaces return.
The FIT file holds the complete recording: a sample per second, the laps, each pool length individually, and everything connected sensors transmitted. It is the difference between a photograph and the film.
What the file genuinely adds
Four families of information exist only in the file, and they change what can be concluded.
- Per-length detail in swimming: stroke count, time and SWOLF for each length, and therefore degradation within a set.
- The second-by-second curve, which allows cardiac drift or normalised power to be computed — impossible from an average.
- The lap structure, which separates reps from recoveries where a global average blends them.
- Data from ancillary sensors: vertical oscillation, ground contact time, left-right power balance.
Without the file, a tool can only reuse averages already computed, and any fine analysis becomes an approximate reconstruction.
The cases where it becomes necessary
The file is not useful every day. Three situations make it indispensable.
- Technical analysis in swimming, where only per-length detail means anything.
- Power work on the bike, as soon as you want normalised power rather than a plain average.
- Recovering an activity recorded by a third-party application, which is not always redistributed through official channels.
- Archiving: the file stays readable independently of any online service.
In every other case the summary is plenty, and far lighter to handle.
How it is built, in two sentences
FIT is a binary format, not readable text. It works by messages: first a definition announcing the fields to come and their size, then the matching data. That is what makes it compact, and tedious to decode by hand.
Two details trap everyone writing a decoder: timestamps count seconds since 31 December 1989, and fields added by third-party manufacturers take up space in every message even when you cannot interpret them. Ignoring them desynchronises the reading of the entire file.
In practice nobody decodes a FIT by hand: you use a library, or a service that extracts what you need from it.
Frequently asked questions
Where do I get the FIT file for an activity?
From the Garmin Connect web interface, an activity can be exported in its original format. A watch plugged in over USB also exposes its files directly, in a dedicated folder.
Can a FIT be opened in a spreadsheet?
Not directly, it is a binary format. It has to be converted first, to CSV for instance, with a decoding tool. Some websites do it online, which means entrusting them with the activity.
Why do some activities never appear in my tracking?
Because activities recorded by third-party applications are not systematically redistributed to partner services through official channels. The session exists in Garmin Connect, but the third-party service never receives it. Importing the file works around that gap.
Can a FIT file also be used to send a workout?
Yes: the format describes both a completed activity and a workout to be done. A workout file dropped into the right folder on the watch appears in its training menu. That is the manual route, with no automatic sync.
Let your assistant write your sessions
Running Workout connects Claude, ChatGPT or Gemini to your Garmin account through the official API. Nothing to install, and no Garmin password to hand over.
See the connection guide