Fitbit cardio fitness is not accurate

My Fitbit charge 4 watch measures what it likes to call 'Cardio Fitness'.  I think its algorithm includes things such as my heart rate across the day, my weight, and I am not sure what else.  I know that it accurately tells the time, it is reasonably accurate when it measures my heart rate, and the Fitbit charge 4 is hideously inaccurate when determining my resting heart rate.  This made me wonder if it was accurate when it determines my cardio fitness.

My watch gives me a score for my cardio fitness, it has determined that anything above 44.6 is very good, and anything above 49.4 is excellent. 

Somehow, according to my watch, when I took that screen shot I was scoring in the 52-56 which is well and truly at the top of the excellent range.  I checked earlier today and am scoring 53-57, which is ever so slightly better.  

Sadly, this is not even remotely accurate.

 

My Cardio Fitness according to Fitbit

The cardio fitness scale it uses is:

<30.2   Poor
30.2-35.0   Fair
35.0-39.8   Average
39.8-44.6   Good
44.6-49.4   Very Good
>49.4   Excellent

All of these increments are 4.8 above the previous.  Another increment above 49.4 would be 54.2.  Given that my estimate is 53-57, at very worse I would be at the very top of the Excellent range (if not somewhere in the next increment above excellent).

My actual cardio fitness, if I am being honest, is pretty low.  

My heart rate is often pretty high even when I am sitting still and resting.  I get out of breath walking up stairs.  While I used to jog for hours when younger, these days I would not be able to run a 5km without having to stop and rest.  This is not an 'excellent' level of cardio fitness.  I think my actual cardio fitness would be somewhat below average.

I think the algorithms that Fitbit uses are based on overweight and obese people.  The fact that I am thin and have a low BMI in the Healthy BMI range means that any cardio fitness results that Fitbit provides are massively skewed.  

I use my Fitbit as a watch, and it works well.  I also find it useful for me to be able to keep an eye on my heart rate.  Fitbit is of no use when it comes to 'cardio health' if you are thin.  

If you are thin, or even not massively overweight, please disregard the cardio fitness level, it would be dangerous to think it was even remotely accurate.