How to join FIT files
- Collect the files. Put every part of the outing in one place. They can be in any order. Between two and ten files work at once. A .zip or .gz can go in as it is. If you're not sure where a file is, the export guide shows where each watch, bike computer and app keeps it.
- Add them above. Drag them onto the box or pick them from your files. FitMerge sorts them by start time, draws the route and shows the totals for the whole outing, along with any warning about a part.
- Pick FIT and save. FIT is already selected. Press the download button and you have one .fit file, named from the start time of the first part.
The reading and the joining run inside this browser tab, and the merged file is saved straight from it. Your files aren't sent to any server.
What a FIT file is
FIT is a compact binary format for fitness data. Garmin's overview describes it as a protocol made for storing and sharing data from sport, fitness and health devices, and it is the format many bike computers and sports watches write when you save an outing. Our tests include real files from Garmin Edge, Wahoo, Hammerhead Karoo, COROS, Suunto and Zwift. Because it is binary, you can't open a .fit file in a text editor and fix it by hand, which is why people look for a tool.
What the merged file keeps
- The track and the sensors. Time, position, distance, altitude, heart rate, cadence, power and temperature of every recorded point come across, in order. Fields a device writes that are not in the public profile are copied too.
- Laps. Each part keeps its own laps. A part with none gets one lap covering all of it.
- The device. The merged FIT carries the maker and product of the device that recorded your parts. That matters for the next step; see below.
- One summary. The distance, time, moving time, averages and maximums are worked out again for the whole activity. The wait between two parts is counted as a pause.
Some things stay behind. Data fields added by Connect IQ apps are left out, and the page warns you with a note on the file that had them. Custom messages that only one vendor understands are not carried over either.
Files cut short, and odd ones
A bike computer whose battery died can leave a file that stops partway. FitMerge reads everything up to the break, merges that part and tells you the file looks damaged or incomplete. A file that never got to write which sport it was takes the sport of the other part, and the page says so.
FIT can be written in several ways, and the files you have may use any of them: big-endian or little-endian numbers, the short timestamps that save space, the older header some Edge models wrote, and definitions repeated before every record. All of these are read. When we tried the same ride written each of these ways, the merged totals came out the same.
Mixing FIT with GPX and TCX
The parts don't have to be the same kind of file. A watch's FIT and the GPX or TCX your phone recorded after the battery gave out can go in together. The result is built in FIT terms, taking the device and time zone from a FIT part when there is one. To get a GPX or TCX file instead, use the Format picker next to the download button. FIT is the one that keeps the device, and GPX is not offered when none of your parts has GPS.
Why FIT is the right download for Strava and Garmin Connect
Because the merged FIT still names the device, a merge that included an Edge 530 FIT was recognised on Strava as that bike computer, and Strava took the elevation from the file. A merged GPX has no device in it. When we uploaded the same merged walk to Garmin Connect as a GPX, it arrived as Other; the FIT arrived as a walk. The step-by-step versions are in the Strava guide and the Garmin Connect guide.
Questions
How big can the files be?
100 MB each, and up to ten files in one go. The largest FIT among our test files is about a hundredth of that size.
Can a watch file and a bike computer file of the same ride go together?
Only if they follow each other. When two files cover the same minutes, the page refuses them as an overlap and names both files, so you can drop the one you trust less.
My parts come from two different devices. Which one does the merged file name?
The merged file takes its identity from the first FIT part, and keeps the device details of every part's device. A GPX or TCX part has no device to offer, so it never takes that place while a FIT part is there.
The page says my file looks damaged. Is it lost?
No. The page merges whatever it could read, up to the point where the file breaks, and tells you which file that was. Check the totals and the map to see whether that is all you needed.
My Zwift or indoor ride has no GPS. Can I still merge it?
Yes. Time, heart rate, power and cadence are all there to join. The GPX choice is switched off when none of the parts has GPS, so save it as FIT or TCX.