How does fitbit know how many calories I burned?

[KF_Ryan360]

I have always wondered how fitbit knows how many calories I burned. Can anyone tell me how?

[SteveH]

There are 2 aspects to calories burned. First there are your BMR calories. These are the calories you burn just keeping your body alive (even when asleep). They are calculated from your profile settings so you needn't be wearing your fitbit to get these. For many people these BMR calories form the majority of their daily calorie burn.

Then there are your activity calories. These are calculated based on the movement of your tracker and how fitbit interprets them. With these your profile settings for age, height, weight etc are all taken into account.

Fitbit have a help file on this here: http://help.fitbit.com/articles/en_US/Help_article/1381

[Dominique]

There are a number of equations that can be used to estimate your BMR (see @SteveH’s reply for details on that). One such equation is Mifflin St Jeor, which uses sex, age, height and weight. You can find the exact equation in this link. Fitbit uses something pretty close. Calories burned through your activity are estimated by Fitbit using the various sensors included in your tracker, and with a clever algorithm. Note that estimating calories with an activity tracker isn’t exact science.

[emili]

And do you have an HR fitbit. Your calories increase as your heart rate increases. If you have an HR tracker it will calculate your calories using HR as part of the equation.

[elderwanda]

I assume that Fitbit also takes into account your body fat percentage, if it has that information (from the Aria, for instance).  Then again, maybe I should not assume that.  Does anyone know if it does? 

[USAF-Larry]

@elderwanda, I don't know that it uses it, but I suspect it uses that calculation as part of the BMR. If you view hover over the Weight tile on the Dashboard, and select See More it will open the Weight log where it shows the Weight, Lean vs Fat, and BMI. The reason those are calculated is because they are part of the BMR.

[elderwanda]

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@USAF-Larry wrote:

@elderwanda, I don't know that it uses it, but I suspect it uses that calculation as part of the BMR. If you view hover over the Weight tile on the Dashboard, and select See More it will open the Weight log where it shows the Weight, Lean vs Fat, and BMI. The reason those are calculated is because they are part of the BMR.

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Ah yes! I had completely forgotten about the Lean vs. Fat chart when I asked the question, but I actually use that information all the time.   I've noticed that in the 7 months I've been working on losing weight (diet and barbell training), my body fat percentage has been going down at a pretty steady rate of about one digit per month (ie 35.4% in August, and 34.4% in September) regardless of how quickly I've been losing actual weight.  During my three week diet break, when I ate at maintenance calories, my weight stayed the same but my body fat went down.  It's really good to be able to see that.   

[Dominique]

I don’t think body fat is taken into account by Fitbit. The equation that uses body fat is Katch-Mcardle. It’s the same for men and women, and isn’t affected by age, nor by height.

The other thing is that body fat would be available only if you have an Aria scale (or another scale that can send body fat to your Fitbit account). And body fat as estimated with bioelectrical impedance analysis isn’t necessary very accurate anyway.