The Apple Watch Stryd app allow Stryders to set a calibration factor. Stryd is accurate out of the box and should not require calibration. To measure actual measured distance is not straightforward, but a fun exercise:
If you run a known distance on a track or similar, you can calculate the calibration factor for the Apple Watch Stryd App:
(Actual Distance/Stryd Distance)
You can set the calibration factor on the watch by going into the Stryd Apple Watch App and scrolling down to the calibration section.
Once you have changed the calibration factor on the watch, the Apple Watch Stryd app reported distance will be different than what is recorded by the offline sync Stryd data.
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