I love my Fenix 6X Sapphire. My only complaint is that it doesn’t recognize the distinction between hiking with a light daypack vs backpacking with a heavy (>30 lbs) backpack for purposes of counting calories. Seems like there should be an option to input estimated pack weight and have that influence the estimated number of calories burned.
The increased weight you're carrying is going to make you work harder increasing your HR which will burn more calories. There is no "x" amount of calories per "x" amount of weight carried.…
The increased weight you're carrying is going to make you work harder increasing your HR which will burn more calories.
Quick test:
- 6 minutes walking with no additional weight
- 6 minutes…
They are relatively comparable. I’m going a little faster without that pack, and I had a little more elevation gain. My heart rates are comparable too. Should they be? Am I actually burning…
does anyone know which method the fenix 6x uses?
Heart rate goes brrrrr method 
No way to know for sure without input from Garmin / Firstbeat. Actually only Garmin, since Firstbeat is owned by Garmin now...
This is dcrainmaker's 2010 matrix; I'm just curious what this looks like now, including fenix series devices.
You're right. What could I have possibly been thinking? Hopefully Garmin will add this calculation. They could add 25 additional calories burned per hour for every 10 lbs of inputted weight. 40 lbs would be 100 additional calories per hour. It doesn't matter if I backpack for an hour on flat surface, or on a 4% incline, or a 1% incline in 10 inches of snow into a heavy headwind. It'll be 100 additional calories added to my workout because it's all the same. 
It's literally a direct function of work you perform on an amount of weight
Exactly. The key phrase is "direct function of work performed". You will burn more calories going uphill as opposed to downhill because of the "direct function of work performed" using the same amount of weight.
shows you have a fundamental misunderstanding of how calories burned is calculated.
No, I don't think you understand. There isn't an infinite amount of calories one can burn per minute. It's based on my output and MaxHR and most experts say that's between 8-15 calories per minute. I've pushed myself to my limit and according to my chest strap that was 12-13 calories per minute. I can reach that limit with or without the addition of carried weight. I'm not going to reach 25-30 calories per min doing the same workout just because I added "X" amount of weight. I'm just going to reach my max quicker. If I were able to walk a mile and maintain a steady HR of 130bpm, I would burn the same amount of calories as if I maintained that same HR while carrying 20 lbs. The only difference would be my pace (work load) was slower with the added weight.. If I were to maintain the same pace (work load) carrying that 20 lbs, my heart rate would increase to meet that demand and I would burn more calories, because it's not the weight carried, but the work load I put forth and that has a limit. Based on your theory, I can exceed any limit by simply adding more weight. You and I could be identical twins. The exact same metrics; height, weight, etc., but you're in better shape than I. If we both did a hike together and carried 40 lbs would our calories burned be the same? No, because you're more fit than I and I had to work harder to keep up with you so my HR was more elevated than yours and I burned more calories. Don't misunderstand me. I'm fully aware adding weight will increase your calorie burn. I'm just saying there are too many factors involved to adding weight to an activity to get a correct estimation that's better than using HR.
Perhaps you should do a simple test: hike an hour with your additional 20 lbs with your normal settings and 1 hr without your additional 2 lbs and check the measurements.
