When comparing calorie data recorded by different devices and their apps, keep in mind that Apple Watch reports active calories, whereas many other fitness devices and apps report total calories (resting + active = total calories).
Calorie estimations depend, in part, on your personal information. To check that this has been entered accurately and to update it over time:
On your iPhone, in the Watch app, go to: My Watch > Health > Edit (upper-right).
To help improve the accuracy of calorie and other activity-related estimations made by Apple Watch when GPS data is unavailable or not relevant, you can also calibrate it:
Calibrating your Apple Watch for improved Workout and Activity accuracy - Apple Support
Resetting your calibration data can be equally as helpful for Series 2 & 3 models and will not erase your Activity history.
Also check your Location Services settings:
On your iPhone, go to: Settings > Privacy > Location Services:
- Check that Location Services (the main setting at the top) is turned on.
- In the list of apps beneath, check that Apple Watch Workout and Nike+ Run Club are set to While Using.
- Also enable options under System Services including Motion Calibration & Distance, Wi-Fi Networking and Mobile Network Search.
For the best performance from the heart rate sensor during workouts, ensure that you wear your Apple Watch snugly (but not too tightly) on the top of your wrist. Apple suggests that you consider tightening the band before starting a workout and loosening it again afterwards:
Wearing your Apple Watch - Apple Support
Other issues that can impact sensor performance include skin perfusion (which can also vary, depending on ambient temperature) and wrist tattoos:
Use the Workout app on your Apple Watch - Apple Support
Use the Activity app on your Apple Watch - Apple Support