fitstitch

Stitch FIT activities that were accidentally split (stopped and saved halfway) back into one, offline and without losing data. The result is the activity you would have had if you had pressed pause instead of saving.

It also trims an activity you forgot to stop at the end (or started too early), as if the watch had been stopped at the right moment.

It works with FIT files from any device (Garmin, Suunto, Wahoo, Coros…), and the stitched file uploads to Garmin Connect, Strava and other sports apps.

$ fit-stitch part1.zip part2.zip -o run.fit
                               Part 1               Part 2             Stitched
Start             2026-10-25 12:39:12  2026-10-25 13:43:54  2026-10-25 12:39:12
Distance                      7.27 km              4.19 km             11.46 km
Timer time                    0:53:51              0:26:08              1:19:59
Elapsed time                  1:04:03              0:27:12              1:31:54
...
Pause between part 1 and 2: 0:00:39

  [ok] file structure and CRC are valid
  [ok] Garmin FIT SDK decodes the file without errors
  [ok] all 16929 data messages preserved
  ...

Use it in your browser

https://jmallach.github.io/fitstitch/ runs the same code in your browser, with nothing to install, on phones too. Your files are not uploaded: the page runs fitstitch locally with Pyodide (Python compiled to WebAssembly). Add two or more files to stitch them, or a single file to trim it on a chart of its speed and heart rate. It is available in 38 languages, from Arabic and Hindi to Catalan and Japanese; corrections are welcome (see CONTRIBUTING.md).

Install

pipx install fitstitch

or pip install fitstitch. For the latest code, use pipx install git+https://github.com/jmallach/fitstitch, or pip install . from a checkout.

Requires Python 3.9+ and the official garmin-fit-sdk package. That package is used for field names and units, and to check the output independently.

Usage

fit-stitch PART1 PART2 [PART3...] [-o OUTPUT] [-n] [-f] [--overwrite]

• Inputs are .fit files, or .zip files holding one (such as Garmin Connect's Export Original). You can give them in any order; they are sorted by start time.
• -n/--dry-run shows the result and the checks without writing anything.
• -f/--force stitches parts even if the sport or the recording device differ.
• If the output fails any check, the tool writes nothing and exits with status 2.

Trimming

fit-trim FILE [--start WHEN] [--end WHEN] [--auto] [-o OUTPUT] [-n] [--overwrite]

• With no options, fit-trim only shows where it would suggest cutting: stretches of at least three minutes at either end spent standing still or moving at vehicle speed. --auto applies the suggestion.
• WHEN is an elapsed time from the start (1:02:30, 45:00, 90s) or a distance (10.4km, 850m); with a leading - it counts back from the end, so --end -23:00 removes the last 23 minutes.
• Records and events outside the kept part are removed. Laps, splits, time in zone and the session summary are recomputed from the records that remain: times, distance, averages and maximums, normalized power, work, and ascent (scaled from the watch's own figures). Values only the watch can compute, such as training effect, are kept and reported.
• The output is checked as stitched files are, and nothing is written if a check fails.
• On the web page, drop a single file to trim it. A chart of speed and heart rate shows where you stopped moving, with handles to drag.

Replacing the original activities

1. Export the original FIT file of each part from your sports app. On the Garmin Connect website: activity → ⚙ → Export Original; on the Strava website: activity → ⋯ → Export Original. Their phone apps can't export files; in a phone's browser, ask for the desktop site if it keeps opening the app.
2. Run fit-stitch (or fit-trim), or use the web page.
3. Delete the original parts wherever they were uploaded. Garmin Connect, Strava and most other apps reject an activity with the same start time as an existing one as a duplicate. Kudos, comments, photos, titles and gear on the parts are lost.
4. Upload the stitched file to Garmin Connect (Import Data), Strava (strava.com/upload/select) or any other app that accepts FIT files. If your apps sync with each other, uploading it to one may be enough.

What it does

The FIT file is read and written byte by byte. Every message and field is copied as it was, including undocumented Garmin messages and developer (Connect IQ) fields. The tool only changes fields whose meaning it knows.

This is the beginning of the README. Read the rest at the original. Read the full README ↗