I would like to know what method is used to calculate active calories burned. Previous to my Apple watch, I used a Mio Alpha heart rate monitor along with various fitness apps. Most of the apps reported similar calories burned based on heart rate. I have never gotten an accurate reading from my watch, regardless of what type of activity I am doing. And yes, I do understand the difference between gross and net calories. The Health app is calculating well below what it should for net calories. I have also made sure the heart rate monitor on the watch is getting similar readings to my Mio. For example, this morning I worked out on the elliptical machine for 51 minutes 42 seconds. I am 54 years old, 5'3 and 144 lbs. My average heart rate for the duration was 146 BPM. The elliptical machine read 546 calories when I was done (I know that is not correct). According to the Health app, I burned 238.59 calories. There is no way that is accurate for a fairly intense workout lasting over 50 minutes. When I go online and look at scientific studies and the calculators used, my gross calories burned should have been ~ 502 calories, and the net calories burned should have been ~ 450 calories. That is a huge difference from what the app is reporting. I am extremely disappointed in how the Health app is performing.