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macrumors member
Original poster
Sep 5, 2019
I would like to calculate my BMR and TDEE using a device such as Apple Watch. So my goal is to get the number of calories I burn with BMR and the "active" calories I burn with exercise. This allows me to figure out approximately how many calories I burn each day and how much food I can eat.
I am well aware that the one proposed by an Apple Watch is an estimate, just as the one that the various online calculators can give you is an estimate. What I would like, however, is as accurate an estimate as possible.
Let's start with the BMR. Here I don't have much doubt, because both the Apple Watch and the online calculators give me indicatively a very similar estimate. So for this I average the result of the calculators with that of the Apple Watch and I am OK.
Different discourse with regard to TDEE, specifically, removed the BMR, what is left is the active calories burned during the day.
I attend the gym 3 times a week doing different weightlifting exercises. I work out about 1h. In this hour, the Apple Watch shows me that I consume 550-600 kcal on average.
Reading online though I see that on average an hour of weightlifting can bring you a consumption of about 250-300 kcal which is about half that, so the Apple Watch would be giving me a 50% error which is not small.
Going then to look at my average heart rate during weightlifting sessions and trying to do some math with online calculators that keep track of calories burned based on heart rate I saw the following. On average I have about 130 beats per minute during the hour of weightlifting training, so according to several online calculators, I consume about 600 kcal, which is quite in line with the data proposed by the Apple Watch.
How can I interpret this data? How much can I trust the Apple Watch on the calculation of active calories? How do you guys deal with these cases?


macrumors 65816
Apr 4, 2017
How much can I trust the Apple Watch on the calculation of active calories? How do you guys deal with these cases?
From my experience over the years I own Apple Watches, the estimated calories burned are quite OK and don’t seem too far off for me.

In general I try to not overthink it, I see most health metrics as kind of a baseline that I keep an eye on and investigate if something is looking odd.
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macrumors 6502
Jun 25, 2014
How do you guys deal with these cases?
Personally, I've found that beyond a certain point, searching for exactness in these sorts of calculations is a fool's errand, and it's best to make your peace with the imprecision.

It's extremely useful to have a general idea of your TDEE within ~200 calories, but beyond that there are so many factors that could influence the calculation. This could be temporary things like ambient temperature, air quality, how well you slept, subtle changes in how you carried out your exercise, your gut health on that particular day, or ongoing things like your genetics and body composition. Likewise with food, nutrition labels are helpful but have pretty big error bars. So unless you're living in a laboratory, there's always going to be a fair amount of guesswork.

For myself, I follow two general principles:
1) double check my calories consumed/expended with changes in my weight. If I think I'm in a consistent surplus for a month, yet I'm losing weight, clearly there is a miscalculation somewhere.
2) Don't eat my exercise calories. I have a daily calorie budget, but I don't see exercise calories as adding to that (though if I go on a longer run - say 10k plus - I will fuel with more carbs). This makes calculating exercise calories less significant. Did my swim burn 300 or 350 calories? ¯\_(ツ)_/¯ I don't know exactly, but either way it was a good swim.
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macrumors 68040
Jun 2, 2008
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