How FitMerge works
Four steps, no black box. At every step you can see what the tool found and change what it does.
Step 1 — Upload and health check
Drop in one or more FIT, GPX or TCX files — segments of one long activity, or the same activity recorded on two devices. Every file gets a health-check report before anything is merged: how many recorded points it contains, the time range it covers, and which data streams are present and how complete each one is — GPS position, elevation, heart rate, cadence, power and temperature.
The health check is also where damage shows up. A FIT file that ends abruptly, fails its checksum, or is missing records a healthy file should have is flagged here with a plain-language diagnosis, so you know about a problem before it turns into a failed upload to Strava or Garmin.
Step 2 — Choose a merge mode
Stitch in time order. For one activity recorded in pieces — a paused computer, a dead battery, a device swapped mid-ride — the files are joined end to end in chronological order into a single continuous activity.
Merge overlapping recordings, stream by stream. For the same activity recorded on two devices at once, the recordings are aligned on their timelines and you pick the source file for each data stream separately: GPS and elevation from the bike computer, heart rate and cadence from the watch, power from whichever device actually had the power meter. Streams a file does not carry are simply taken from the one that does.
On Pro, the combination you pick can be saved as a stream profile, so next time the same two devices record together, the same choices apply in one click.
Step 3 — Repair (when a file is damaged)
If the health check flagged a damaged FIT file, FitMerge attempts a repair before merging: recovering the recorded points from a truncated file, correcting or working around CRC errors, and rebuilding a missing session record from the data that is actually there. The repair report lists each problem found, what was done about it, and what could not be recovered.
Repair cannot invent data that was never recorded — a section of the ride the device never wrote stays missing, and the report says so. What repair does is make the surviving data into a valid file that other platforms will accept.
Step 4 — Download and reconcile
Download the merged activity as GPX or TCX (free on every plan) or as FIT (Starter and Pro) — the format bike computers and Garmin use natively, and the one that carries the full set of data streams.
Every output comes with a totals reconciliation: the output's point count, distance, duration and per-stream coverage, set against the inputs. The numbers should add up the way you expect from the merge mode and stream choices you made — and if anything was lost, the reconciliation is where you see it, before the merged file replaces your originals.