Calorie descrepencies between MyZone and Charge 2

[vegansummer]

Hello,

I've recently been given a Charge 2 for my birthday and have only used it a couple of times at the gym for workout classes.  I already had a MyZone (MZ-3) belt and have successfully been able to track effort, heart rate and calories burned on both the gym monitors and on my mobile phone app.  Today I used both the fitbit and the belt and the amount of calories burned during a vigorous spin fit class differed wildly between the two devices.  The MyZone output indicated I had burned 450 calories whilst the fitbit showed only 162 calories.  This is not an insignificant difference.  The class was 45 minutes long and my heart rate is around 150 with an effort level mostly between 80-95%.  Which one is likely to be more accurate and why are the two devices so hugely varied?  Is there something wrong with the fitbit?  Has anyone else had similar issues?

Many thanks in advance.

Sarah.

[MarreFitbit]

It's nice to see you around, welcome aboard!  I've found great posts in which some of my friends share all their knowledge about how the calories work and how to better understand them. Said that, check this post of @SebringDon about calorie burn accuracy, this one of @PureEvil about calories burned during exercises, and this one of my dear @MariamV about how to calculate calories burned from exercise. Also, since your Charge 2 tracks your heart rate I'd recommend checking this post to know when and how is your heart rate used to calculate your calories burned. 

Hope they help!

[vegansummer]

Thanks for your email.  Unfortunately none of the recommended posts have answered my question.

[Ukase]

 MyZone is allegedly 94% as accurate as an EKG. Not sure what Fitbit's stats are, but odds are, the MyZone is more accurate. That said, consider that when you first try on a Fitbit - presumably you've already loaded your age, height, weight, etc. With MyZone - it sets you up with a resting heart rate of 100bpm, and adjusts from there. Usually in a day. So - the calorie reading you get on that first demo isn't a good comparison. Additionally, MyZone uses your max heart rate to determine what "zone" you're in when you're exercising and attributes your calories based on that percentage of max heart rate. 

Suppose you're in great shape for someone your age? MyZone isn't going to know that for another month or two, when it detects that you might be staying in the 90-100%% range longer than you should be able to, and it will adjust your max heart rate upward. These variables will certainly impact your total calorie results. 

I noticed that Fitbit overestimates walking/running compared to MyZone, and Fitbit underestimates other non-walking/running workouts. 

The truth likely lies somewhere in between.