I have a fitbit one and it seems pretty accurate for me as did the ultra I had previous. For me it does sometimes miss steps though it depends on the activity but very soft fluid steps are sometimes missed i.e. steps in some dance classes I took (hula, flamenco). When I test the step counting by starting the timer then counting each foot fall or weight shift I take and walking 100 or 200 steps around the house--it is usually a couple steps under. With jogging it usually around the right step count. With jumping it counted a step per jump but got a few extra. So it can vary a bit but it has been pretty good. I haven't had the One over count for me at all.
With distance, well, I calibrated the stride settings because I wanted my outside walking distance estimates to be as accurate as possible. Now the distances are often accurate or at least within .25 of a mile (often within .1) compared to GPS app, known distances, or treadmill distances. It can vary depending on my speed and circumstances but it is usually good. The distance doesn't really effect the step count or calorie burn though, it is more if you care about that sort of thing. with my settings when it is off it is usually a little low not high. With the defaults it use to be overestimated distances for very brisk walks--I think it was crediting me with running for brisk walking sometimes so I set my running stride a little short.
With calorie burn. This is really hard to say because any device we use is just providing an educated estimate. I once compared 3 estimates for walking one mile on a flat track at a consistent speed of 4 mph. I had warmed-up first with a mile of walking and jogging intervals (not really a warmup but I wanted my heart rate elevated before I started the test). I compared my heart rate monitor/GPS app, Fitbit and this online calculator that I was told was pretty accurate: http://www.exrx.net/Calculators/WalkRunMETs.html
The results: Fitbit had the highest calorie burn (slightly) it was 95 calories. My heart rate monitor the lowest with 91 calories. The calculator was right in the middle with 92 calories. I know this was less than an hour--I figured roughly over an hour fitbit and my hrm would have about 20-30 calories variation for that activity at that intensity. I am okay with that. Distance on this was weird--both my GPS and fitbit were a little off on the distance compared to what the track posted as the distance. The GPS was .1 of a mile over and fitbit the same amount under! But .1 mile is a pretty small error and again, close enough.
I've had years of comparing my fitbit and HRM calorie burn for different activities. For me this is how they compare:
Fitbit estimate higher than HRM: Walking if my heart rate is not in aerobic zones (commonly for walking my heart rate seems to be 80-100 bpm on flat ground) in this case fitbit is closer to the calculator I posted than my HRM. HRM's are calibrated specifically for aerobic activity so I am not sure what to make of that. And this is a weird one--elliptical machine going as fast as I can but using no resistance at all. I just did this as a test and only once so I don't know if this is consistently so, my heart rate was in moderate aerobic zones the whole time. Fitbit credited it as if I was running very fast but it for some reason was not as tiring as running (I suppose the lack of impact).
Activities where Fitbit and HRM match or are similar (it may vary which is higher): brisker walking if in aerobic zones as mentioned in the 4mph test, jogging, cardio kickboxing workout dvds like Turbojam, jumping rope, aerobic dancing with a lot of traveling steps, some aerobic drills type workouts with exercises like jumping jacks i.e. "Banish Fat Boost Metabolism".
Activities where the HRM was much higher than fitbit: any kind of resistance training, rowing machines, cycling or spin class (100 calorie difference for a one hour class this is my biggest variation), stair laps (this surprised me as my One usually counts the floors very well), yoga.
Note some of the activities that fitbit underestimates compared to the HRM are also activities the HRM cannot track well--yoga, strength training, etc. Pretty consistently my hem/fitbit estimates are lower than the activity databases on Spark and Fitbit. I tend to log the HRM calorie burn when different but it is adding less than it would if I used the database.
Oh floors, this seems least dependable to me. It seems fine when I do stair laps in my building -- 5 floors up then 5 down. It usually counts all the ups but will sometimes miss one out of the five. This compounds if I do something like 20+ floors up it can end up losing 2-3 floors. I never see it overestimate for the indoor laps. Windy weather and other weather changes can make it count extra floors outside though. I am pretty sure they are not factoring in floors for the calorie burn due to what I noticed with stair laps.
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