How does the Aria 2 measure body fat percentage?

[Kieran9619]

Hi, 

First things first, I'm a lover of my Fitbit charge 2, and I believe the aria 2 would be ideal for compatibility of measurements. However, I'm a sports scientist and also a personal trainer and I believe that the scales are to some extent overpriced, believing the weight to be accurate, BMI figure should be obvlivious for 95% of the population due to it's inaccuracy of measurements it take into account. But my question really is If anyone can answer, how does it measure your body fat percentage? And how accurate can that be? Because a bioelectrical impedance analysis cost cheaper and has a more detailed and structured procedure. I understand Fitbit wants to be the Apple IPhone market for sport, however the description doesn't provide significant enough detail for me to buy this for it's cost. Someone please help.

Moderator edit: updated subject for clarity

[Baltoscott · Accepted solution]

@Kieran9619 -- The Aria 2 uses bioelectrical impedance to estimate your fat percentage.  As far as accuracy, it depends on how similar your body composition is to the group of people they must have used to create their algorithm.  The premise is that fat conducts electricity at a different rate than the rest of your body (water, muscle, bone).  It will likely be slightly different from other scales using bioelectrical impedance (I have an Aria 2, a Withings scale, and an Omron.  I can get on and off all three within a couple of minutes and they all give different fat % results).

Although the absolute number might not as accurate as, say, hydrostatic weighing or a DEXA scan, it is a lot more convenient, and assuming you weigh yourself the same time of day as close to the same hydration level as possible (hint, right after you wake up), it is good at documenting whether you are getting more or less lean, not just whether you are getting lighter or heavier.  

As far as price goes, there are cheaper alternatives.  But a huge plus for fitbit users is that it automatically logs to your fitbit account giving you the ability to see progress (or lack thereof) over time.  

[JuanJoFitbit]

@Kieran9619 Thanks for participating in our Fitbit Community! Also thanks a lot for your interest in our new Aria 2. The information that shared my friend @Baltoscott is totally correct. That is the way that body fat percentage is measured. I would like to follow up and would like to know if you need more assistance.

See you guys later! 

[CraigG20815]

Question for anyone who might know:  My Aria 2 seems to be way off in calculating my body fat percentage (it consistently says that I’m at 35% or more, which seems unlikely).  I would be happy just to ignore that.  But (unless I am imagining this) it now seems to think that I am burning calories at a much slower rate than it did before I added the Aria.  Is that right?  Just a coincidence?  Any solutions to these problems?  Thanks!

[JuanJoFitbit]

@CraigG20815 Thanks for joining us in this thread! Seems odd that your Aria 2 is not calculating your body fat accurately. The Aria scale measures your body fat according to your personal info (weight, height and gender). Please check out this information on your profile settings through your web-based Dashboard. If the information is correct and the issue persists, please take out the batteries for 15 seconds and put them back in. After this, weigh yourself 5 consecutive times in order to re-calibrate your scale and see if your body fat percentage gets better accuracy.

Keep me posted on the outcome my friend! 

[Baltoscott]

@CraigG20815 — I agree with @JuanJoFitbit That you might want to reset your scale to see if that affects your body fat percentage.  (If accurate, 35% body fat percentage for a guy would probably put you in the obese category for BMI — body mass index — which would be 30 or higher).  

But I think your question is really more about whether Fitbit takes body fat into account when calculating your BMR (basal metabolic rate) which affects the calculated calorie burn.  According to what I can find, it only seems to take gender age weight and height into account when calculating BMR, so the body fat percentage should not affect the calculation of your calorie burn.  Maybe @JuanJoFitbit can confirm?

[CraigG20815]

Thanks @Baltoscott and @JuanJoFitbit.  The recalibration did work, and brought the body fat percentage down to about 21%, which seems more in line with being at roughly normal weight for my height.  @Baltoscott was right that I was curious whether the body fat percentage played any role in the calculation of BMR.  Guess I’ll now see if the rate of calorie burn changes.  But if @JuanJoFitbit knows the answer to that, I would certainly be curious to hear about how the underlying magic is done....  Thanks much, both!

[qtpieprnsz]

I have had my scale for 5 days and am now seeing what everyone is talking about.  Body Fat % seems to go up and my weight goes down... not sure why, but when you take into account the cost of the scale, and the integrity of FitBit, I think FitBit should do some sort of update to rectify this.  Surely with technology today it has to be possible.  For a large percentage Aria 2 users to be experiencing this tells me something is off in the scales 'brain'. 

FITBIT people, can you advise if you have a team looking into rectifying this?  Thanks and good luck to all on your journey!!

[Baltoscott]

@qtpieprnsz -- you noticed that "BF% seems to go up as your weight goes down," over the five days you've had your Aria 2. That makes sense when you realize that the primary reason for daily weight change is a change in your body's water retention (your body is mostly water, after all).  

Basically a scale like the Aria 2 tries to divide your body weight into two parts based on a small electric current running up one foot to your bottom and down to the other to close the circuit: body fat and everything else -- muscle and other non-fat tissue, bone, water -- collectively called lean body mass (LBM).  Your LBM will move a lot throughout the day and from day to day as you eat and drink, process food and go to the bathroom.   In a calorie deficit your body fat will decrease over time, but whatever you see from one day to the next is mostly water.  So if you dropped two pounds of water since yesterday, but the fat was mostly the same or only went down a few ounces, then, mathematically, your BF% would go up.

A better measure of whether your body fat (or weight) is going the direction you want is to look at weekly averages in the app.  As your average weekly weight goes down your BF % will tend to go down too because more of the weight loss will be due to fat loss.

[qtpieprnsz]

OK, thank you for the additional information.  I will aim to gauge it weekly and see how my BF % goes throughout this time!!  🙂

[Baltoscott]

@qtpieprnsz — cool.  Keep weighing in every day at about the same time of day and you will get more consistent results over time.  I weigh in first thing in the morning, before eating and after going to the bathroom.

[Cath661]

I am having the same issue.  I only weigh once a week, so how will it affect the weekly average?  My Body Fat is going up each time I weigh instead of down, even though I've lost weight.

[Baltoscott]

@Cath661 - since you only weigh in once/week your weekly averages will the same as your weekly weigh in.  Some weeks you’ll be more or less hydrated which will affect both the weight reading and the body fat calculation, but over time as your weight continues to go down so will your body fat.

[TrudyC]

Hi there, I’m having the same issue! It’s saying my bmi is 35%! Im pretty slim, in no way over weight and based on my height and weight everything I read says I shouldn’t have a bmi this high! 

I’ve  tried recalibrating as suggested. I’ve also tried disconnecting the scales altogether and setting them back up again. 

Not sure what else to do, any suggestions? 

[LZeeW]

@TrudyC  I had a similar issue when I got my scale.  On my own, I decided to change my setting to the LEAN mode, even though my athlete days are over.  I got a much more reasonable percent body fat.  It is worth a try.

I did an internet search for images of different percent body fat.  I look like the number my scale says now.

For clarification, you seem to be confusing BMI and percent body fat.  BMI is a calculated relationship between height and weight.  Percent body fat -- which is what you are referring to -- is that % number that your scale measures.

[AndreaFitbit]

Welcome to the Fitbit Community @TrudyC. I apologize for the delayed response and will be glad to assist you with your body fat inquiry. Thank you for mentioning that you re calibrated your scale and also disconnected all together and set up again.

Do you exercise and have an established work out routine? If you do,  I would recommend @LZeeW advise and set your information to the Lean mode. 

Please let me know how it goes. 

Thank you for your advise @LZeeW. Keep on visiting the forums. 

[Kmilacesar]

I do exercise and diet and everytime that my weight drops, the bf rises. escale is showing something around 27% right now and there is no way that I am.over 22%, it is not only a huge gap but also what upsets me is the fact that i can clearly see that im loosing fat, but the escale just increases this number.

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[JuanJoFitbit]

@Kmilacesar Thank you for joining us in this thread! Since you are experiencing the same issue with your Aria 2, I would like to know if you have tried the recommended troubleshooting steps. If so, please let me know what you have tried. This will help me to assist you accordingly.

See you later! 

[Kmilacesar]

yes I did,
I tried to remove the battery and replace it again and weight myself several times.
I tried to change to lean mode.


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[Baltoscott]

@Kmilacesar — were you successful in changing it to lean mode?  I assume the android app is similar to iOS — the slider should indicate that it is on (mine is not). Lean mode not on

I did use lean mode for about 3 months last year because I think the normal mode reads a little high for me compared to other estimates, but it came in MUCH lower (went from about 18% to less than 8%).  After doing that for a while I just decided to switch back.  It may read a little high for me (and for you), but it trends well if you measure every morning like I do and trends are what are most important to me.  A downside of my use of lean mode for a few months last year — and then going back to normal — is that I don’t have good BF% comparison numbers as I cut to similar weights this year (I went through a bulk/cutting cycle in an attempt to build more muscle).  So my suggestion is that if you do decide to switch to lean mode leave it there so you can compare same to same over time.