How are kilocalories calculated?

Question

This week I have received much lower kcal scores for runs. For comparison: 
 Until recently my usual 3-4 mile runs, recorded kcal like this: 5km: 388 kcal 4.8: 380 5.3: 430 
 But this week my two runs so far have been given kcal like this 4.87km: 65kcal 6.47: 101 
 I&rsquo;m at a loss to understand this. All the data: heart rate, pace, etc are comparable. I record these on a Garmin Forerunner 735xt. 
 The obvious thing is that my weight might have changed in GC, but no, it&rsquo;s the same.

trux · Answer

There are some differences in the HR, for example more irregular HR with higher spikes, and more time spent in Z3 at the activity with higher Calories, though I am not sure it could cause such a big discrepancy in Calories. 
 There are some suspicious facts though: 
 First of all I see the HR throughout the day is measured only during the activity, so I wonder where the Resting HR comes from, because normally the watch measures it during the sleep, or during resting phases. Did you set it manually, or did you wear the watch with the HR sensor on, the other days of the week? 
 And then, the Performance Condition is completely missing at the activity with the low Calories. That typically means, the HR did not reach 70% of the Max HR for a sufficiently long time during the first 20 min. And that could point to the Max HR being too high. Did you really verify the Max HR value in the device settings &raquo; User Profile? ( Not on the Max HR Report!) 
 I recommend wearing the watch with the HR sensor on 24/7, setting the Max HR in the user profile to a known value (either from a lab or from a field test) and disabling its auto-detection, force-rebooting the watch (sync, then press the Light button for some 30s till it completely shuts down), and then doing a few runs again.

How are kilocalories calculated?

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