How Does the Apple Watch Calculate Calories for Workouts?

Okay, maybe I’m a bit obsessive, but when I spend $400 on a device, I expect it to perform as advertised. Early today, I showed that the Apple Watch confuses miles and kilometers, and doesn’t count distances for indoor walks very well. In the past, I’ve also pointed out how there are problems with the heart rate sensor, and that the device’s resting calories are from the realm of science fiction.

After a friend read the article about miles and kilometers today, he said, “78 calories? That’s weird.”

Apple watch miles kilometers

I had never actually considered whether the calorie counts were correct or not, though I’d heard some people report very odd numbers. So I went to an online calculator, entered my weight, the grade of my treadmill (measured with the Compass app on the iPhone), the distance the treadmill recorded, and the time. It told me:

You burned an estimated 230 calories.

Now, that calculator is counting gross calories, or what the Activty app displays as Total calories (Active plus Resting). But there’s still a huge discrepancy: 144 for the Apple Watch, 230 for the calculator.

The same website also has a calculator that uses your heart rate. Since the Apple Watch records this – albeit not very accurately – I went back to a 20-minute outdoor walk I did last week. I entered the time, the average heart rate, and the rest of the information, and the calculautor told me:

At a heartrate of 109 bpm, or 64% of your estimated maximum heartrate (170 bpm), your calorie burn is an estimated 646 calories per hour. In 19 minutes and 43 seconds you will burn approximately 212 calories.

The Activity app says I burned 83 active calories, and a total of 128 calories.


It’s worth noting that the type of workout you choose seems to affect the calorie count. If I walk on my treadmill, I can choose either an Indoor Walk workout or an Other workout. Here are the calorie counts for both kinds of workouts, for 30-minute walks at 3.5 mph in the past few days:

Indoor Walk: 91
Other: 179

I think that Apple needs to do some work on this whole fitness tracker thing. It’s hard to take any of this seriously now.