Now that I’m on a journey to reduce my body fat percentage, I need better measurements to assess my progress. I’m still skeptical of digital bathroom scales that claim to measure body fat, but is there anything better?

I decide to use a “smart” scale, and I select the Fitbit Aria2 because the Fitbit app is where I’ve put all of my manual weight-tracking measurements for the last few years. I want my data to be all in one place, and that makes the vendor lock-in powerful.

The Aria2 looks great and is easy to use, but the accuracy left something to be desired. The weight was exactly in line with my other scale, which is to be expected. However, the body fat measurement was way off compared to the DEXA scan I had earlier that day. The DEXA scan said I have 29.4% body fat, but the Aria2 says I only have 14.7% body fat. Clearly one of these measurements is very wrong.

I contacted Fitbit support with my concerns and was instructed by the support agent to try putting my profile into “lean” mode (ha!).

Aria 2 has two modes to calculate body fat. The default is “regular” mode, which applies to most people. This mode works well for people who don’t get a great deal of exercise as well as those who exercise frequently. “Lean” mode is intended for professional or very high-level athletes like marathon runners or body builders. Individuals with exceptionally low body fat relative to their muscle mass may want to consider lean mode; however, many very fit and active people get accurate readings in regular mode. - Fitbit Support

Although that clearly does not describe me, I figured I’d try it anyways. When I did, the answer I got back was 12%, which is probably even more wrong! Further measurements all came back between 12 and 15%, so the Aria2 isn’t even accurate with itself between repeated weigh-ins on the same day.

I asked Peter from BodyComp for his opinion about the wide difference, and his reponse was more interesting:

Bioelectrical impedance assessment for determination of percent fat is an interesting animal. It’s attempting to measure your fat percentage based upon resistance to the flow of a series of electrical pulses. However this pathway is essentially up the leg, through the groin and down the other leg. As you can imagine, this pathway doesn’t always represent the make-up of the whole body very well.

So my working theory at this point is that the Aria2 is giving me a different result because my legs are relatively more musclar than my abdomen or upper body.

Overall the Aria2 is a nice looking unit, but it is terribly inaccurate when it comes to Body Fat percentage. I’ll be disabling display of this metric. Maybe it will get more useful over time?