figuring out calories burned using a HRM

I'm expecting Santa to bring me a heart rate monitor (strap that goes around your chest with a watch).  I'm sure there are directions with it, but since I do not yet have it in my possession....how do you determine calories burned during a 24 hour span using one of these contraptions?  Do you plug in your weight and all that jazz in the watch part or is there some mathematical formula to use?  Just curious. 

To be honest, I'm not sure it would be accurate enough for the 24 hr period.  I have a Polar F6 that I use for every workout, but from what I understand, when your heart rate dips below 80-100 bpm, they aren't as accurate.  I've honestly never worn it 24 hours a day but I do recall some posts of ppl who have.  You may want to contact the manufacturer once Santa brings it (provided they're not chillin' at the North Pole).

I'm not sure either.  To be honest, I'm not sure how accurate it will be in general.  The one I'm getting has good reviews but who really knows how accurate anything is.  Maybe I'll start off by just wearing it during my workout.  I've had my BMR tested professionally so i pretty much know what I burn during a 24 hour period at sedentary.  I would like a more accurate portrayal of what my workout burns though since I don't go by calories burned on any cardio machines.