Hi
Apple Watch uses different sensors and data sources to track the results of workouts, depending on the activity type.
For example, when recording treadmill-based walks or runs, the built-in apps and the Nike+ Run Club app track your arm motion (measured by the accelerometer) when estimating results including distance and pace.
For the best results, allow the arm on which you are wearing your watch to swing naturally as you move around and during treadmill-based workouts.
Calibrating your Apple Watch can improve the accuracy of activity-related estimations. Calibration teaches your watch how your arm movements relate to your stride lengths at different speeds when you are walking and/or running. It does this - during outdoor walks and/or outdoor runs recorded via the Workout app - by comparing data from the accelerometer with GPS / Location Services data.
More information and instructions for calibrating your Apple Watch is available via the link below. The article also includes instructions for resetting existing calibration data and starting afresh. Resetting your calibration data will not erase your Activity history.
Activity-related estimations also depend, in part, on your personal information. To check that this is accurate - and to update it over time:
On your iPhone, in the Watch app, go to: My Watch (tab) > Health > Edit - tap and adjust items, then tap on Done.
More information: