The Apple Watch and Resting Calories

Update: Since I wrote this article, Apple has changed the way the Apple Watch and the Health app calculate BMR. It is no longer the ridiculously high amount I mention below; for me, it is around 2,200 calories each day, but it still varies.

I’ve highlighted how the Apple Watch isn’t very accurate as a fitness tracker, and I’ve shown that my Apple Watch’s heart rate sensor doesn’t work correctly, which is leading Apple to exchange it.

But one thing I find surprising is the way the Apple Watch – or, more correctly, the Activity app on your iPhone – calculates resting calories. This is another term for basal metabolic rate, or BMR, the amount of energy your body expends just to keep you alive. If you did absolutely nothing during a day, other than sleep, your body would still burn a certain number of calories. In fact, your active calories only represent a small part of the amount of energy you use.

Like many such measurements that are difficult to measure, the BMR varies according to the way it’s calculated. But one such calculator tell me that may BMR is 2008 calories. Another one, at myFitnessPal, tells me my BRM is 1894 calories.

Resting caloriesNot Apple. According to the Activity app, my resting calories for the full day yesterday was 3184, or 50% more than a BMR calculator. As such, the Activity app tells me that I burned 3829 calories yesterday, with 645 of these being active calories. Yesterday, I took two brisk walks: one on my treadmill, for 30 minutes, which counted as 178 calories, and one outdoors, for about 20 minutes, which clocked 83 calories.

It’s interesting that the 20-minute outdoor walk only counted for about half as much as the 30-minute indoor walk, which measured calories based on my heart rate, and, presumably, the frequency of my steps.

No matter how you slice it, these numbers are wrong. I’ll give Apple a pass on the active calories; there’s no way to get those numbers down precisely. But the BMR, or resting calories? I’ve entered my data in the health app – age, height, weight and sex – so, unless online calculators are way off the mark, Apple has some tweaking to do.

What’s also surprising is that this number isn’t the same every day; it ranges from 3172 to 3195. This is a fixed number, that has nothing to do with my activity. There’s not a big difference between the top and bottom of the scale, but they should be the same every day. Only the active calories should change. (Update: this is because my weight changes slightly every day; I use a wireless scale which syncs its data to HealthKit.)

And this is particularly worrisome. This number is based on a very simple calculation, and isn’t skewed by the way you move your arms, or your stride when you walk. It’s the one number that they should get right.