Caloriescalculated through HR data, steps, or both?

[Tt11]

I am interested in how fitbit calculates total calories burned. I understand it is BMR + exercise calories as I have seen on other answered questions.

However, for the exercise calories burned, is HR data, steps taken, or both of these used to calculate it?

The reason I am asking- tracking calories burned is very important to me. I live on a farm and drive tractors, atvs, trucks etc and often get MANY fake steps since there is no quick and easy way to pause step counting. If step count was used to calculate calorie burn I would be very alarmed as to the inaccuracy of my data. 

Please respond! Thank you 

[JohnnyRow]

Calories burned are based only on heart rate, not steps, aside of course from the fairly constants such as weight, age, etc.

[Tt11]

THANK YOU!!! What a huge relief!