How does Aria 2 calculate body fat % ?

[stegag]

I was always confused about how my body fat % is calculated.

I always felt like the % is not a current measure, but is software-calculated against previous measures.

It seemed like too much related with the previous one.

Today i've done a test and weighed myself multiple times in a row. These are my result:

Time - weight - body fat%

10:14am - 86.5Kg - 23.2% fat

10:13am - 86.5Kg - 23.1% fat

10:11am - 86.4Kg - 23% fat

10:11am - 86.5Kg - 23% fat

10:10am - 86.5 Kg - 22.9% fat

10:09am - 86.6Kg - 22.8% fat

10:08am - 86.5Kg - 22.7% fat

Don't you think this is a bit weird? obviously i didn't eat anything meanwhile...

How is the body fat % calculated and which measure should i consider reliable? i don't trust the device anymore.

Moderator edit: updated label

[SteveH]

I think the variation is just down to it being an approximation. Fitbit say:

"Body fat percentage is the mass of your fat divided by your total body mass. It's estimated using bioelectrical impedance analysis, which sends a small, safe signal through your body"

From: https://help.fitbit.com/articles/en_US/Help_article/2223.htm

There's a good description of how this works in wikipedia: https://en.wikipedia.org/wiki/Bioelectrical_impedance_analysis

[stegag]

i get that and i get the approximation.

obviously there can be measurement errors but i'd expect them to be constant, or at least random throughout the measures (like 0.1Kg going up or down, it's totally fine).

But here the fat% goes up or down after every measure too linearly.

[SteveH]

I agree it's weird. I can't explain it. Hopefully someone else will post an answer. 

Out of interest did you try another measurement a little later e.g. an hour, to see if it continued to rise or went back to nearer the 22.7% ?

[stegag]

i have weighed multiple times today: every measure is now constant on the 23.2%.

the i have deleted all measures from yesterday and today, and it now says 22.6 %

whatever 🙂