I calculated the accuracy of my Fitbit calorie burn measurements

[Pigufleisch]

I use Fitbit to track my activity and MFP to track my intake and I couldn't live without either of them.

While it is easy to try to improve how accurate my food tracking is, it always bugged me that I didn't really know how accurate my Fitbit's measurements were. Since "Calories In - Calories Out = Weight Loss", getting an accurate reading of Calories Out is really important.

So I sat down and figured out the difference between what my Fitbit says I burned and what was probably true in reality. I'm pleased with myself so I thought I'd share it here, too, in case it gives any of your the kick you need to do the same for your readings.

Before I talk about the steps I did to calculate my fitbit inaccuracy, keep in mind that I am assuming that (1) my scales and the system I use to weigh myself are both perfect, and, (2) my food intake tracking is perfect. In reality, the inaccuracies are due to everything in this process and not only my Fitbit but, since I'm already doing my very best with food intake tracking, it's easier to modify my exercise to change the overall result.

I used a spreadsheet and wrote down "Cals burned" (I'm assuming that this is TDEE) and "Cals eaten" weekly for every report I have received. I used the spreadsheet to calculate my caloric deficit each week. I then did a quick google search and used the popular rule of thumb that 1 pound = 3500 kcal to calculate my theoretical ideal weight loss each week based on the deficits measured by Fitbit.
I then wrote out what my actual weight loss was. Using those numbers, I figured out what my actual caloric deficit really must have been each week. Finally, that allowed me to figure out my actual total "Cals burned" figure. Comparing my fitbit results versus these numbers, I discovered that Fitbit is giving me a result for Calories burned by activity and metabolism that is, on average, 15.79% overestimated.

Obviously that 15% inaccuracy might not actually be due to my fitbit flex. But it does mean that I can use this knowledge to perfectly stay on track to my weight loss goal by only eating back 85% of my fitness calories. Knowledge really is power (and that's why I have a fitbit). Feels good.

TL;DR - I geeked out and used my fitbit weekly reports plus maths to discover fitbit is 15% overestimating cals burned.

[musiclogic]

I thought I was the only freak who would spreadsheet my way to more accurate results. (Look at me, I just used spreadsheet as a verb!)

Good job. I have been meaning to do the same but because I have lost weight pretty consistently, I figure I can let this one go, although I crave accurate data.

I posted a while back somewhere in this forum the math (based on my own pace) to determine if people with super high step counts were cheating or not. I felt that some of the huge stepcount numbers were achievable as a one off challenge sort of thing but not achievable consistently unless the person had no job or no life, or perhaps their job is walking for a living.

Anyway, thanks for posting. Good work.

[Pigufleisch]

Thanks!

I've been losing weight consistently, too. I wasn't do this so much as to be able to gain control and lose weight at all. It was more for the fact that I've been doing so well I felt inspired to take ever more control over the process. There is just so many variables and complex interactions in diet, fitness, exercise, weight loss etc that being able to cut out some unknowns feels really good.

Since I posted this originally, someone else put me on to a fitbit partner programme, Trendweight, so now my fitness geekery can hit even greater heights.

Regarding the super-steppers, I wondered about that, too, and found that I could keep up with some of the big steppers for a while but I was killing my body and especially my feet over time. I've decided to leave them to their Professional Walking and do what works for me 

[Heybales]

During a 2 month period with some pretty decent training going on, in which all my exercise was manually corrected because Fitbit was woefully underestimated, my result based stats showed 5% over.

And this was with an already increased height stat so that the BMR calories used during non-moving time would match a better estimated BMR/RMR based on bodyfat %, not just age, weight, height.

This included weighing all foods, perhaps 3-4 meals were rough estimates eating out or such.

But anything within 10% is actually really good, considering people can force their own systems slower by undereating by upwards of 20%, and some have burned off enough muscle mass through yo-yo diets that their metabolism is slower than average calc's would figure.

[slysam]

Pigufleisch, Were you logging any activity during your test? I did similar with my One a couple years or so ago. I was logging my nonstep workouts (using my heart rate monitor for the alternate even though some were activities a HRM can't accurately estimate calorie burn for). I was weighing food, except when I ate out or at someone else's house. I probably had about 3 meals a week that I was guestimating--not weighing portions and even guessing what was contained. During that time, I was typically losing a pound per 4,000 calorie deficit on Fitbit. I felt that was pretty good given that several meals were guessed and the HRM has a margin of error too. Later that year, I experimented with not manually logging any exercise. At that time several of my workouts were non-step activities like strength training, yoga and pilates. Well, that month I lost weight faster than my Fitbit deficit. But this was a One.

[Pigufleisch]

The only activity that was being logged was either walking or running. I haven't got back into my gym routine lately so, in terms of accuracy, I was only doing things that the fitbit is good at tracking. I log all my food meticulously on MyFitnessPal and while I know there will be some inaccuracy in that I do not think that I can improve there. It's more valuable in practice for me to just figure out the variance of the entire system and just change how I exercise accordingly. 15% more!

[slysam]

Oh, I don't log walking or running usually (unless there are significant hills). I just let Fitty track it.

[whome]

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@Pigufleisch wrote:

Thanks!

I've been losing weight consistently, too....... There is just so many variables and complex interactions in diet, fitness, exercise, weight loss etc that being able to cut out some unknowns feels really good....

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Well, as someone once said, "There are known unkowns and there are unknown unknowns"

Good luck

[Heybales]

What about the known unknowns!

The method is actually pretty decent, even if you are terrible with logging foods.

Because if it appears you are eating say 1800, but you know full well your logging is sloppy and inaccurate, and you are actually eating more than 1800 by some unknown amount, and you only lose 1 lb in 2 weeks, or a 250 cal deficit daily basically, then all you have to do is find an accurate 250 cal to stop eating daily, and your loss can of course become 1 lb weekly.

But I would suggest this method only works if you are coming in from the high side for sure.

Trying to work your way up from minimal calories while barely losing doesn't work nearly as nice.