Tldr: I'm crazy and gave Apple all possible data to ensure maximum accuracy of the apple watch calorie burn calculator. When done, what the apple watch calculates my total calories as matches my spreadsheet for maintenance and to within about 30 calories of what cronometer says I have actually averaged eating.
Two weeks ago I got an Apple Watch series 4, and I’ve been testing how accurate it is when it comes to calculating total calories.
I've completely blown away at how accurate the calories count is from the apple watch.
Starting 2 years ago I lost 137lbs and have been in maintenance for a little while now. I have as much as possible weighed all food and drinks to the gram while recording them in cronometer. I've also tracked my weight and calories every day in a spreadsheet to to see my daily average tdee. In addition I've recorded body measurements weekly to have an estimate of body fat percentage. At some point I also started using the app happy scale to record my weight daily. I use the app Gymaholic which uses the Apple Watch to calculate calories burned while strength training which is then recorded as part of active calories burned according to apple, and that app also has my measurements to estimate body fat percentage. Plus my doctor's hospital has an app that can sync with apple health so that that data is there too. I also took the time to calibrate the apple watch to my step candence for maximum accuracy of step counts. Finally I wear my Apple watch to sleep and track using the app AutoSleep.
I gave apple health full access to all apps containing this data that I could, and I inputted whatever I couldn't into it's records.
Basically Apple has my gender, age, height, weight, body fat percentage, body measurements, all vitals as recorded by a doctor, medical conditions, medications, activities, around 23 hours a day of heart rate monitoring, sleep pattern, caloric intake, and all major and most micronutrients taken in from over a year.
I get how scary that sounds, but I long ago accepted that between apple and google I have no private data so it isn’t a big deal to me.
I don't know what data apple uses when it calculates calories burned since that equation is proprietary. I'm guessing it doesn't actually need all that information, but I'm mentioning it because Apple has it and there are many tdee calculators that have increased accuracy when they are given additional data. My guess is that apple probably only needs height, weight, gender, age, as much heart rate monitoring as possible, and maybe body fat percentage for full accuracy.
Anyways, using this tdee calculator I have a BMR of 1349.
Now realize that apple calculates resting calories. This is different than BMR. BMR is the calories you'd still burn even if you were comatose. Resting calories seems to includes certain basic things like additional calories from minor movements throughout the day. For all intents and purposes it is calculating sedentary levels.
So when I sleep, I burn an average of 57.5 calories an hour according to my Apple watch. If I slept a full 24 hours that means the apple watch says I would burn 1380 calories a day. This matches my BMR. Since resting calories includes more than BMR, I actually average 1627 resting calories a day according to apple. According to that calculator I linked I would have a sedentary tdee of 1618. Again, this matches.
Now for active calories, apple says I burn about 1012 calories a day on average based off the mix of what apple watch is recording for my daily movements and runs plus the calculations from gymaholic for strength training. So I have a daily average tdee of 2639 according to Apple.
Meanwhile based off my spreadsheet I burn 2645 calories a day on average. Based off cronometer I have eaten an average of 2615 calories a day. Finally, happy scale, when the daily fluctuations are averaged out, shows that my weight is at maintenance without any losses or gains.
All this is to say, if you give apple literally all needed biometric data (whatever that is), wear your Apple watch as close to 24 hours a day as possible, use tracking apps to update your biometrics daily, and a workout app for calculating strength training, the apple watch appears to be accurate within about 30 calories of what I actually consume.
If you don't give it as much information, then I’m not sure if it would maintain that level of accuracy. Still, it's kinda amazing to realize just how accurate it can be.
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