Calories burned MFP vs heart rate based

onikonor

I don't have a HRM (I did order one just waiting for it in the mail) and decided to take my heart rate manually during a workout. I did circuit training for about 40 minutes and took heart rate during peak (manually) which was about 180 bpm (does this number make sense?). When I keyed it into the calorie calculator it gave me net calories burned of 460 while mfp gives 390.

My first question is how can people get into 700+ calories burned range? I worked really hard and got only to 400+.

Out of curiosity, which one do you think is more accruate? Which one should I enter?