What goes into calculating Calories?

[Zharko]

I'm trying to understand what goes into calculating calories. At a minimum, I'm sure it's a byproduct of at least the following: age, gender, height and time. 

In math speak:  f(age, gender, height, time elapsed in day) = calories

Can you clarify what else is used? Does Fitbit also consider a person's weight? what about HR values? (probably but want to confirm) Is XYZ acceleration (i.e. the raw movement data) used to influence calories as well? Are there any other inputs which haven't been mentioned? I'm not looking for the 'secret sauce/function,' I'm just trying to understand the rough influencing factors that go into the 'calories' time series and under what scenarios I would expect devices set up differently to produce the same or different results.

[SteveH]

Fitbit's (fairly limited) guide to this is here:

http://help.fitbit.com/articles/en_US/Help_article/1381

So, there are 2 elements to consider. The calories you burn through activity and the calories you burn just keeping your body alive (your BMR calories).

Activity calories depend on the movement your tracker detects and, for certain fitbits, your heart rate information. These are influenced by your profile settings for gender, age, height, weight.

Your BMR calories are purely based on your profile settings so don't even have to be wearing your fitbit to get them tracked.

[Kevin_H]

See also here: https://dev.fitbit.com/docs/activity/ (discussion under Get Activity Intraday Time Series subheading):

Using BMR/EER Algorithms

If a user had no Fitbit tracker data for the specific day then the greater of Logged Activities + BMR (for minutes when there is no activity) and the calories calculated from the EER for that day (if EER enabled for this user's profile) are taken. In case, there was some data from the tracker for the specific day, that data where available is used and for time where data is unavailable, the BMR is used. If the total is less than 20% greater than BMR then the EER (cals < EER * 0.8) is used. EER never used to calculate calories for today.

Using BMR Formula

Fitbit uses the standard MD Mifflin-St Jeor equation:

9.99 * weightKg + 6.25*heightCm - 4.92*ageYears + s, where s is +5 for males and -161 for female

EER Formula (TEE total energy expenditure)

The EER Formula is based on http://www.cdc.gov/pcd/issues/2006/oct/pdf/06_0034.pdf, which in turn is based on "Food and Nutrition Board. Dietary reference intakes for energy, carbohydrate, fiber, fat, fatty acids, cholesterol, protein, and amino acids (macronutrients). Washington (DC): National Academy Press; 2005." http://www.nap.edu/openbook.php?isbn=0309085373&page=204

MALE-based EER Formula:

TEE = 864 - 9.72 x age (years) + 1.0 x (14.2 x weight(kg) + 503 x height (meters))

FEMALE-based EER Formula:

TEE = 387 - 7.31 x age (years) + 1.0 x (10.9 x weight(kg) + 660.7 x height (meters))