Graft heart rate, power, or cadence from one file onto another.

No problem - the tool matches by absolute timestamp, not by record index. As long as the two files overlap in real time, the graft works. The portion of the target outside the source's time window stays without grafted data (you keep whatever was there originally).

5 seconds is a sensible default and covers most cases. Drop to 1-2 seconds if both devices recorded at 1Hz and you want strict matching. Raise to 10-30 seconds if one device subsampled its stream (some Garmin units record HR at lower frequencies to save battery, and some power meters report at varying intervals). The diff stats after merge tell you what percentage of target points matched, so you can re-run with a larger tolerance if too many points were skipped.

No - only per-trackpoint stream values. The target file's laps, session summary, course points, and event messages stay intact, so the structure of the activity does not change. This is intentional: you usually want the target's organisation and the source's sensor data, not a mash of both.

Two-device recording. You record an outdoor ride on your Garmin Edge (which has GPS, power, cadence) but you also wore an Apple Watch (which captured HR and recovery data). You want one merged FIT for Strava that has the Garmin's power and the Apple's HR. Drop the Garmin FIT as target, the Apple GPX as source, check the HR box, download. Done.

Yes - if the power meter records its own FIT (some do via Connect IQ data fields or a paired phone app), drop the no-power head-unit ride as target and the power-only FIT as source, check power, merge. The output will look like a normal head-unit recording with power baked in.

Yes. The "target" and "source" labels are just which file the output is shaped from. You can use either file as either role. Try one direction, look at the diff stats, and switch if the other way is cleaner.

The graft overwrites the target's existing values at each matched timestamp. This is intentional - you usually graft because the target's version of the stream is missing or wrong. If you only want to fill gaps (write source values only where target is missing), check the gap-fill option on the tool page.

Yes - the output is a fully valid FIT (or GPX/TCX) re-encoded with the official Garmin SDK. Strava treats it like any other upload. If you are re-uploading because the original was missing a stream, Strava will detect a duplicate by start time and distance - either delete the original first, or shift the start by a minute using our [FIT Time Adjuster](/tools/fit-time-adjuster).