Percent body fat

[Doc_Yuengling]

I had tape measured myself for an accurate body fat percentage in January when I entered it with a daily weight.. I did this three more times this year, but with an estimated body fat %, using the waist, neck sizes with age and height factors for the USMC system on the internet. I am 5' 6" and now about 156 pounds, relatively the same weight for the last 6 months, but with higher muscle mass.  

I noticed as I have been doing high intensity CrossFit, and cardio since April, I would put on weight..

Focusing on overall weight due to my BMI of 24.9 - 25.3, I kept the proteins, but cut total calories slightly to slowly loose a few pounds to maintain weight, as opposed to rapid weight loss. My anecdotal goal to stay in the normal BMI range for height and weight.

As I went back to look at body fat percentage, which I had not looked at in three months, my Fitbit dashboard decreased my body fat percentage from more than 13% to earlier in the year, to 9.3%, without any input over the last three months. My lowest calculation was 12.7% previously, 3 months ago.

I would have assumed that it would require an input of some kind. 

Does anyone know how the Fitbit calculates this factor?

I do use a Fitbit scale with a Bluetooth connection for daily weights first thing in the morning. 

I had previously entered a body fat % only 4 times in  the last year (about every 3 months), and not in the last 3 months. 

[JohnnyRow]

Some scales measure/estimate body fat % by bioelectrical impedance.  If such a scale syncs with Fitbit, the body fat % probably also syncs.  I believe that is the only way Fitbit gets body fat, other than manually entering it.