Hiking calorie count inaccurate - Fenix 6S Pro

7117605 said:

I walked continously for 8 hours the total calorie burn displayed was only 1537 kcal (942 active and 595 passive). This number seems ridicously low compared to (i) my previous walks with my Polar watch and (ii) considering the effort put in for the body. But most importantly, when I look at the daily recap, the app says I walked 40,2km/54.783 footsteps that day (= hike + trips going and back to the hike departure and arrival not recorded) and here it says all these footsteps represented 2811 kcal which seems way more accurate. If I do the math based on the footsteps calorie count, the activity calories should have been of 2600 kcal and not 1537!

The value of 2811 Calories on the second screenshot is the total number of Calories burned during the whole day (24 hours), and not only for the steps. It includes also the RMR (Resting Metabolic Rate) for the entire day, which can be in fact often higher than what you burn actively. 

I do not know what you RMR is, but let's say 1700 kcal (you can verify it on your Calories page). In that case it would mean that on 16th July you burned 1700 Resting Calories + 1111 Active Calories (942 during the hike, and 169 during the rest of the day). Adjust the numbers by substituting the RMR with your true RMR.

In other words, on the first sight it looks all right and credible, but you would have to post also the graph of your HR during the day, to see the effort, and your Resting HR.

Ok, it got me confused as the 2811 kcal data was located under the "Steps" tab, but when I go to the calorie control panel and to that date, it says indeed 2811 kcal for the day (without specifying anymore if it's linked to the steps I walked or not). That day my passive calories were 1753 (can't seem to find the RMR if it's any different than passive calories...).

Here's my HR graph which I find low because I didnt walk very slow that day, and I'm usually easily towards 100 BPMs when walking a bit fast. 

Maybe this is linked or not but I realized that the 1st BPM zone was set by default to 90 if I recall, and I read somewhere in the Garmin's help notice that below that 1st zone, BPMs are not taken into account. Could this have changed anything to the HR data and the resulting burnt calories ?

And finally, my Samsung Health app estimated the total calories burnt at 2130 kcal for that distance. How can this be so different even if not based on HR?

7117605 said:

That day my passive calories were 1753 (can't seem to find the RMR if it's any different than passive calories...).

Yes, RMR and Resting Calories (or if you want "passive" Calories) are the same.

7117605 said:

Here's my HR graph which I find low because I didnt walk very slow that day, and I'm usually easily towards 100 BPMs when walking a bit fast. 

The daily HR graph would have been better, because there we could see also your Resting HR (RHR). You get Active Calories only for the time when the HR is higher than the RHR. And the higher the HR is above the RHR, the more Calories you get. Hence, if your 7 day average RHR is high, and the HR during the activity is low (like in this case), then it means there was only low effort, hence low energy consumption (low Calories). The number of steps is irrelevant. What counts is the effort, and that is measured by the HR.

Here's the HR graph over 7 days (but I've only had the watch since July 14 afternoon, so 6 days today excluding my gym session set for tonight):

So according to you, it is normal that the watch counts the same total calories burnt when walking 40km at a rather low pace and when doing 100min of high cardio session (av. BPM 165)? (this is the case if I rely on the watch)

7117605 said:

So according to you, it is normal that the watch counts the same total calories burnt when walking 40km at a rather low pace and when doing 100min of high cardio session (av. BPM 165)?

Yes, it sounds possible. Hard to tell exactly without calculating it precisely, but generally a 100 min high intensity training can very well burn similar amount of Calories as 8 hours of low intensity activity.

7117605 said:

And finally, my Samsung Health app estimated the total calories burnt at 2130 kcal for that distance. How can this be so different even if not based on HR?

If Samsung Health calculates the Calories only from steps, then it does not include any other effort during the day. It only takes the RMR (perhaps even just the BMR in case of Samsung) for 24 hrs + Active Calories for the steps during the day. The result is some 700 kcal lower than what your watch shows. First of all, because calculation the Calories burn from steps is only very approximative, and then because the total amount does not include any other activity. You burn energy not only by doing steps, but with any other physical and mental effort during the day (including during the measure activities). So I'd tell the results are pretty close, and well within the acceptable tolerance, considering how little accurate the calculation at Samsung Health is, and few input data it has.

I found some old Polar calculation for hikes of similar average BPM and speed and it is no comparison to Garmin. On the Polar, by applying the same ratio, the total amount would have been of 2700 kcal and not 1500. 

My guess is that Samsung Health only estimates activity calories and does not make an assumption of global energy expenditure for the whole day. For the measurable activity done, Samsung says I burnt 2100, vs 1500 on the Garmin. Massive gap...

PS: sorry I can't figure out how to quote

I do not know how Samsung or Polar calculate the Calories, and I am not going to study it, so cannot tell whether they are right or wrong. The method Garmin uses is documented here:

• Firstbeat Gets Calorie Counts Right - Firstbeat
• Energy Expenditure Estimation – Firstbeat White Paper 

Besides assuring the HR is being recorded right, and entering the right personal data into your profile, there is not much you can do to influence the calculation. On some watches the option Activity Class may play a significant role on the way Active Calories are being calculated, and since you wrote it was not set, it might be one of the reasons the results were low. On newer models, the parameter is not supposed to be taken in view, though.

Read more on the topic here (never mind it is for a different model, most of it is generally valid, and the French version may be better for you):

Qu’est ce qui est utilisé pour calculer les calories sur ma montre de la série vivoactive ? | Garmin Support