How to accurately calculate calories burned in exercise??

I'm just entering maintenance and now I will need to be eating back all my exercise calories, but I feel like it's hard to know what's accurate. I have a heart rate monitor, but it always seems to spit out higher calorie burned amounts than what I get when I take the average heart rate the HRM gives me and plug it in to an online calculator such as http://www.shapesense.com/fitness-exercise/calculators/heart-rate-based-calorie-burn-calculator.aspx or http://www.calories-calculator.net/Calories_Burned_By_Heart_Rate.html. MFP's numbers are way under what I get calculating by average heart rate.

For reference, I'm 5'5'', 26 years old, 128 pounds. I'm pretty fit, but not a marathon-runner or anything and my heart rate still gets up there (175-180) when I exercise, but always comes down immediately when I stop. (recovery rate = 25-30)

Is calculating calories burned by going off my average heart rate is most accurate? yes or no?

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