How Does AW Calculate Resting Calories?

Discussion in 'Apple Watch' started by JPM42, Aug 21, 2016.

  1. JPM42 macrumors 6502

    Oct 21, 2007
    I'm increasingly interested in buying an AW next month with what could be the AW2. While I'm sure I'll appreciate the other features, most of the purchase is based on fitness, and just trying to get more information than the step tracker on my iPhone.

    Anyways, I'm curious how the AW calculates resting calories. In other words, it seems when you first set up the Activity app, you enter in your age/height/weight/gender, etc. I'm assuming that's done to calculate your BMR. However, after that, when you're prompted to specify your activity level, does your answer get baked into the resting calories? Or does your activity level only set your daily calorie goals?

    It seems when the watch first came out, it had a lot of issues with resting calories that weren't resolved until watchOS 2. So I'm curious, at this time, what the default behavior is.

  2. exxxviii macrumors 65816


    May 20, 2015
    I have not seen anything where Apple has explained how the AW uses the biometric inputs to calculate a BMR. So, I suspect that your suppositions are correct: that it uses the age/height/weight/gender to estimate BMR like everyone else. I do not know what it does with activity level. I cannot remember where the setting is and how I could change it now.

    The original OS1 did have some major BMR estimation errors for a lot of people. The BMR was about 1,000 high. That got fixed in one of the OS1 point releases.

    The calorie estimation algorithms still contain some errors and seem off relative to other algorithms on other devices. I suspect that this is tied to the original BMR errors with the legacy code still throwing off the calculations for activities. You can do a little deconstruction math against the active and resting calorie numbers from the AW to see the errors. My hunch this will get a little better with OS3 and better yet with OS4.
  3. BlueMoon63 macrumors 68020

    Mar 30, 2015
    It is now as accurate as any basic measurement for resting calories that is gets from you about your stats. For me, it is spot on or equal to any measurement formula. +/- 5%


    Like BMI, it won't know your muscle mass or lack of muscle.
  4. midloman macrumors regular


    Feb 10, 2010
    Midllothian, VA
    I second BlueMoon63.

    I was one of those folks that was having massive issues with the calorie calculations in the first version of the Watch OS. I have absolutely no issues with how it's being done now; it's more in line with other trackers I've used before. I'm not sure what they changed or how exactly they're using the information they have, but it seems to be working fine now. I'm pleased with things.

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