You can upload up to 25 files at a time via the Strava website. Strava accepts .tcx, .fit, and .gpx files that are 25 MB or smaller.
Save the files you wish to upload to a local folder on your computer.
Log in to Strava.com and click the orange "plus" icon at the top-right of the screen. Choose "Upload Activity" and then "File" from the menu on the right-hand side.
After clicking "Choose files", browse to the local folder where your files are saved, and select the files you wish to upload. Subscribers can upload up to 25 files at a time. Athletes who are not subscribed to Strava can upload 15 files at a time.
