13-30-3 workout on treadmill, big gap in calories burned on the watch vs the treadmill

Hi Mariephan,

We recommend you try the steps in the link below to improve your Workout and Activity accuracy.

Calibrate your Apple Watch for improved Workout and Activity accuracy - Apple Support

To perform the recommended troubleshooting steps via the link above, complete the steps in the "Calibrate your Apple Watch section."

1) While wearing your Apple Watch, go to a flat, open outdoor area that offers good GPS reception and clear skies.

2) If you have Apple Watch Series 2 or later, you just need your Apple Watch.* If you have Apple Watch Series 1 or earlier, bring your iPhone for GPS. Hold your iPhone in your hand, or wear it on an armband or waistband.

3) Open the Workout app, then tap Outdoor Walk or Outdoor Run. To set a goal before starting, tap .

4) Walk or run at your normal pace for about 20 minutes.

If you don't have time to finish the workout, you can complete 20 minutes over multiple outdoor Workout sessions. If you work out at different speeds, you should also calibrate for 20 minutes at each of the speeds that you walk or run.

Whenever you walk or run outside using the above steps, your Apple Watch continues to calibrate the accelerometer by learning your stride length at different speeds. Calibration can also improve the accuracy of your calorie calculations in the Workout app, and the calorie, distance, Move, and Exercise calculations in the Activity app. 

We recommend the steps above because they will show you how to calibrate your Apple Watch to improve the accuracy of your distance, pace, and calorie measurements.

Please let us know the outcome after giving the above steps a try.

Thanks for using the Apple Support Communities.

Cheers!