My experience was the opposite. I had a Fitbit Versa, and over a full year of activity tracking and meticulous food tracking, I found its daily calorie burn number to be very accurate, like within 100-150/day. By contrast my Active 2 is just plain laughable. Like, fall-out-of-my-chair ridiculous. It's showing under my actual burn by 1000-1500 calories/day. It assumes I'm either sitting still, or walking slowly. The heartbeat accuracy is another matter; it'll show me at ~100 sitting in my chair but taking my pulse with my fingers and a stopwatch I'm closer to 50 (my known and medically-verified resting pulse). I really like the rest of the features on this thing and the calorie tracking isn't that important to me any more... I'm just surprised they've bungled this part so bad given how well the rest of it works.
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