Calories burned during runs are suddenly inaccurate

The calories burned during some activities have gotten increasingly inaccurate. I track my runs on another app which, in the past, normally lines up with my Garmin. The Garmin calories have been steadily decreasing and today it showed half the number of calories burned as my other app. It shoes I burned 400 calories on a 9.5 mile run. The calories for cardio workouts have also been inconsistent. What can I do to fix it?

  • What can I do to fix it?

    Have consistent HR during the activity, and have consistent average resting HR. The later requires that you wear the watch during sleep (and have indeed a good sleep), and/or during resting periods of the day (sleep is better though). If the average resting HR suddenly changes, the Calories burned during an activity will change too.

  • My heart rate is consistently at 150 bpm the entire run.  And my resting heart rate is also consistent through out the day.  I wear the watch 24/7.  It was working fine til a few weeks ago and I  haven’t changed anything.

  • OK, in that case please post a direct link to two comparable activites demonstrating the discrepance (please set the privacy on them to Everyone - see the lock icon for that purpose), and post also a screenshot of the average resting HR for given period, from the section Reports, so that we can have a look where the problem is.

  • Well, comparing activities from two different applications makes no sense, because I have no idea how the other application calculates the energy expenditure. Please post two similar activites from Garmin Connect, where there is a serious difference between them.

  • Just trying to show you how off it was today.

     Here are three 9-10 mile runs, all from the Garmin app, over the past few weeks.

  • I'll need also the resting HR data for those weeks

  • OK, here you go. It took me an hour of my free time, but what wouldn't I do for a fellow athlete. Hope you appreciate it, at least.

    I entered the data of all three activities, and the resting HR for the respective days, into an Excel table. I do not know the exact algorithm from Firstbeat that Garmin uses for their Calories calculations, so I used a commonly available formula for calculating Calories from HR (source: Quora). I just did a minor change, since I do know that the Firstbeat algorithm uses the resting HR, hence I implemented it into the formula too. Then I took just a wild guess, and did the calculation for a female athlete 30 years old and weighting 50kg.

    The resulting Calories using the common formula from Quora are in the column calc Cal, and the Calories reported by Garmin Connect are in the column GC Cal. The ratio calc/GC is then in the last column.

    As you can see yourself, the values match almost perfectly for all three cases (margin of error of only -1% to +2% !). Frankly told, I did not even expect that perfectly matching results, because I believe the calculation Garmin does, is much more complicated, using aso the time in individual HR zones. They also certainly do not use the average HR for the entire activity time, but rather aggregate the energy expenditure over the entire time. That explains the slight difference of the results from the simplified algorithm I used.

    The conclusion is simple and clear: Garmin is right, your other app has it wrong. And that for the simple reason that they most likely do not calculate with the measured effort (the HR), and only estimate the Calories from the distance and time, instead.

    PS: The formula I used is shown below, including the RHR inclusion (41 + [@[Δ HR]])). It certainly needs some tuning to match better the true age and weight of the athlete, but I am not really willing to spend more time on it. I believe that as the evidence it is more than sufficient.

    ((-20,4022 + (0,4472 * (41 bpm + ΔHR)) - (0,1263 * 50 kg) + (0,074 * 30 years))/4,184) * 1440 * activityTimeInMin

  • My goodness, talk about going above and beyond!  But you’ve helped immensely because now I can easily see that it’s the heart rate monitor that’s off.  I’m testing it now and it’s definitely having a problem.  I’ll call Garmin on this because it is likely an issue with the device itself.