Charge 3 cardio fitness information too low

[Fiafries]

Hello, I'm a healthy, active person who is in a healthy weight range. My resting heart rate is of 60, which I thought was pretty good, however, my cardio fitness is 37, which is average. I go out for runs 2-3x per week and I workout very often as well. 

Why is my cardio fitness so low? It hasn't improved at all in a few months, so I'm pretty sure there's a problem with my device. I've tried resetting my Charge 3 and uninstalling the app, but neither of these have helped. 

Thanks!

Moderator Edit: Clarified subject

[AlejandraFitbit]

Welcome to the Community, @Fiafries.

Thanks for troubleshooting this cardio fitness issue. Your cardio fitness score is determined by your resting heart rate, age, gender, weight, and other personal information. For best results, make sure your weight is correct in your Fitbit profile. Also, wear your tracker or watch to sleep for a better resting heart rate estimate. Your cardio fitness score will be shown as a range unless you use GPS for runs.

Let me know the outcome.