Formula inputs for swimming calories on Charge 6

[marmoset]

Does anyone have any ideas/guesses/actual facts about how the calories are estimated?

In general the number seems close enough, but randomly (maybe 10% of the time?) it's wildly off.  

In the below screenshots (two per swim session since not all the data is available on one screenshot), the slightly harder workout (longer distance, higher heart rate, much more time in vigorous) is 146 calories, and the other is 934.  If it weren't able to track heartrate or something I could maybe understand the difference (although 146 is barely above basal rate), but it seems like everything is tracked correctly and there's some stat that is not shown that massively impacts estimated calorie burn.

Thanks!

Moderator edit: edited title to clarity.

[CrisVillaFitbit]

Hello, @marmoset., welcome to the Fitbit Community.

Thanks for letting me know about this doubt you have in regards to the calories tracked by your Charge 6 while swimming. I appreciate the information provided.

Your Charge 6 combines your basal metabolic rate (BMR) whic is the rate you burn calories at rest to maintain vital body functions (including breathing, blood circulation, and heartbeat, and your activity data to estimate your calories burned. If your device tracks heart rate, your heart-rate data is also included, especially to estimate calories burned during exercise. The number you see on your Fitbit device is your total calories burned for the day. Maybe you have doubts about the information shown in your app, you can check more about how your Charge 6 calculates this activity here.

[marmoset]

Hi Cris,

Thanks, I'm aware of how it does the calorie estimation, the part I'm confused about is why two almost identical workouts have such different estimates (146 vs 964).