How are calories burned calculated by Garmin watch? (Read 60 times)

How exactly does a gps watch with the green heart rate reader calculate/estimate your calories burned during a long run? It keeps your weight on file, obviously tracks your total distance/pace, and estimates heart rate with a decent amount of accuracy. Does it take into consideration all of these factors or just certain pieces? I have a Garmin Forerunner 45 if that makes a difference.

Last week I ran 8 miles in 64 minutes with an overall average heart rate of 166 and the watch estimated I burned 1033 calories. I let my wife borrow my watch for a long run this weekend which she did 5 miles in 68 minutes with avg HR of 164 but the watch estimated she burned 1066 calories. We are both obviously skeptical about this and it matters to her since she is running to lose weight and doing Weight Watchers monitoring points.

Obviously you burn more calories running 8 miles vs. 5 miles in the same amount of time. She weighs about 10 lbs less than me as well so that shouldn't be a factor and her HR was high for the pace since she is a beginner. Is the watch just estimating based on avg HR for the total amount of time?

I can answer this at a high level.  The very short answer is that you cannot share the watch between individuals because an extremely important parameter for calculating your calories is your fitness level.  The results your watch gave your wife are no surprise to me.  The activities are very similar.  About the same duration and about the same heart rate.  But in both cases the watch basically thought it was YOU.  But the truth is your wife has a lot slower pace for a similar heart rate (she is objectively less fit than you are) -- she's not expending as much energy per heartbeat as you are.  So, to get good calorie results, the watch needs to be used by 1 person all the time so your fitness level estimate (VO2max) can be calibrated to you and stay there.  I hope that's enough information to help out.

Best regards, Joe