FatCal, Fat % and Carb % are determined by which zones you were in during your workout. Ultimately, the total number of carb calories will depend on your heart rate throughout the workout as related to your VO2 max. In lower heart rate zones, you’ll tend to burn a higher ratio of fat to carb calories, while in higher zones, you will burn more carb-based calories.
Currently Fitdigits only does the calculation on the completion of the activity, it does not support real-time calculations.
You can view equations used at the bottom of ShapeSense’s article.
How Fitdigits Calculates Calories
CPM – Calories Per Minute
Fitdigits Calories Integrating with Fitbit Calories
Editing Your Calories, Distance, Duration
Fitbit has recently announced some changes to the way they count user’s active minutes.
Before if at any time you picked up your pace Fitbit would count that increase in effort as “Active” minutes, and simply sum up those minutes each day. However, in compliance with suggestions by the American Heart Association, Fitbit will only count active minutes after 10 minutes of continuous motion. This change will not affect your step count.
As a result, you may find that your Fitbit active minutes will be lower for the foreseeable future.
For more details, read the blog post Fitbit Extends Minimum Time Frame for Active Minutes.