The Apple Watch comes with a variety of health and fitness features, including the Workout and Activity apps. Out of the box, the device uses your personal information (age, height, gender, and weight) to calculate a variety of metrics for your workouts and daily activity.
One of the benefits of the Apple Watch is that you don’t need to bring your iPhone with you when you go for a walk or run outdoors. The device is able to quite accurately track distance without the assistance of your iPhone’s GPS.
However, you can take the time to calibrate your Apple Watch to improve the accuracy of these measurements. Below are the steps required to calibrate your Apple Watch (via Apple Support):
1. Bring your iPhone and your Apple Watch.
2. Find an open, flat area outside that offers good GPS reception and clear skies.
3. Make sure that Location Services is on. To check the setting on your iPhone, tap Settings > Privacy > Location Services.
4. Make sure that Motion Calibration & Distance is on. To check the setting on your iPhone, tap Settings > Privacy > Location Services > System Services.
5. Hold your iPhone in your hand, or attach it to your body with an armband (preferably) or waistband.
6. Open the Workout app on your Apple Watch, and choose Outdoor Walk or Outdoor Run.
7. Choose your goal, or select Open, and tap Start.
8. Walk or run at your normal pace for about 20 minutes.
Apple says that if you are not able to accumulate 20 minutes of activity during one session, you can accumulate 20 minutes over multiple sessions. Users do not have to do anything specific to start the calibration process except having your iPhone without when you start a workout.
By doing this you will calibrate the accelerometer and the Apple Watch will learn your unique stride lengths at different speeds. With time this will help the Apple Watch become more accurate, allowing it to give you better feedback.
Apple notes that calibration data is stored locally on the Apple Watch and isn’t backed up to your iPhone.