Heart rate and calories

[Francies]

Hey FitBitLovers!
I have the fitbit blaze since 2 month (best decision ever). But there are a few functions I don't understand.
What's the difference between heartrate function "on" and "automatic"?
Aaaand I'm really curious about recording calories. Because I thought, the fit bit blaze (or surge, or charge HR) calculates the daily calorie consumption by measuring and calculating the heartrate with your personal fitness and body information you registered in in the beginning. I wasn't wearing my blaze yesterday afternoon and forgott it at home. In the evening I came back and it showed that I burned about 1300 calories in like 4 or 5 hours.
So was I wrong the whole time? Or in the morning, also when I'm not wearing the fitbit at night, it says I burned about 400 calories since midnight. Is the fitbit just calculating the calories and not measuring it like I thought? Because if it is so, the numbers might be not really accurate.
Thanks for your help!

[StealthNinja]

I believe the colories featured are extrapolations of your resting energy consumption as it takes a lot of energy simply to keep the body alive even when you are sleeping and sitting.

[JohnRi]

You tracker is using the BMR fomula to calculate how many calories you burn, regardless of activity.  My BMR is 1850. So if all I did was lay in bed all day, I'd burn that many calories. That's why you see 400 burned since midnight when it resets.  Your body constantly burns calories.  You have to breathe, pump blood, keep warm... 

In addition your tracker adds more calories burned based on step counts, and your heart rate.  So as you move around and exercise durning the day, your calories burned goes up.

Is it 100% accurate?  No, it's just an estimate.  But on a day when I walk 6 miles compared to a rest day, the calories burned is very different, as it should be.