Is the Fitbit calorie burn accurate?
By Coach Hunter Bennett
In this article I explain why using your Fitbit to track burnt calories may not be the best idea.
Wearable activity monitors have taken over the fitness space. Whether it be Fitbit, Garmin, or apple, it doesn’t matter. They are popular, and they give you an abundance of information to keep track of.
With one of the most popular being their “calories burnt” function.
But should you be relying on them to measure the number of calories you burn during exercise?
How does Fitbit calculate calories burned?
While I will be talking about the Fitbit a lot in this article, what I am discussing is applicable to all physical activity trackers.
It is just that Fitbit is arguably the most popular, and as such, have the largest body of research behind them.
Now, how does the Fitbit calculate calories burned?
Before anything else, Fitbit uses an equation that integrates your height, weight, gender, and age to estimate your basal metabolic rate (or BMR for short).
Your BMR can be considered the amount of energy you burn to survive.
Every day your lungs use energy to breathe, your heart uses energy to pump blood around your veins and arteries, and your cells use energy to transport molecules throughout your body. All of which requires energy.
It is this that makes up your BMR.
Once Fitbit has calculated your BMR, they combine it with changes in your heart rate to estimate the amount of energy you burn during exercise.
Note that I have emphasised the word estimate in both stages.
This word is important.
See, at no point does Fitbit measure your calorie expenditure. This is something that can only be done using a laboratory-based gas analysis system. Instead, it relies on generic equations to provide a best guess of the calories you are burning.
But does it work?
Is the Fitbit calorie burn accurate?
A recent meta-analysis (a study that combines the results of other studies) answered this question.
They combined the results of 29 studies comparing Fitbits estimated calorie burn against a measure of calories burnt and found the average association between these measures was 0.64.
A perfect association would be 1.00 – so 0.64 is not ideal.
I appreciate that this number doesn’t really offer much real-world context, which is why I wanted to dig into a couple of the studies included in that review in more detail.
The first sought to evaluate the accuracy of Fitbit to a measure calories burnt during aerobic exercise, and found they:
- Overestimated calories burnt by 26 – 61% during walking.
- Underestimated calories burnt by 37 – 59% during cycling.
- Overestimated calories burnt by 25 – 39% during jogging
The second sought to evaluate the accuracy of Fitbit to a measure calories burnt during weight training, and found that they:
- Overestimated calories burnt by 12 – 48% during weight training.
As you can see, the ranges are large. And this can have a big impact.
Why you shouldn’t use Fitbit to measure calories burnt
The reason this is an issue is because many people track their calories to help with weight management. They might use their calories burnt during exercise to guide dietary decisions later in the day.
For example, let’s say someone is trying to lose weight. They might go do weight session, and their Fitbit tells them they burnt 500 calories. With this information, they might decide to eat a bit more at dinner, because they believe they will be still under their calorie target for the day.
However, chances are they didn’t burn 500 calories.
In fact, it is much more likely that they burnt something closer to 300.
Meaning that the extra calories they eat could put them out of a deficit, stalling weight loss. Then combine this lack of progress every day for a year, and they could give up on their goals completely.
Similarly, if someone is trying to lose weight and have chosen cycling as their form of exercise, they might grossly underestimate the amount of energy they are burning. They might think that they need to do hours of exercise to burn even the smallest amount of energy.
This could lead to A) them doing an absurd amount of exercise and overtraining, or B) giving up because it all seems too hard.
Neither is a good option.
Is Fitbit Calorie Burn Accurate: What to do instead?
Fortunately, there is a better option than tracking your calories with your Fitbit (or any other activity tracker, for that matter).
- Calculate your maintenance calories (click here)
- If your goal is to lose weight, consume ~10% less than this number per day.
- If your goals is to build muscle, consume ~15% more than this per day.
- Aim for 1.6–2.2 grams of protein per kilogram of bodyweight, per day (read more about how much protein to gain muscle HERE).
- Eat 0.5–1 .5 grams of fat per kilogram of bodyweight, per day.
- Have all your remaining calories coming carbohydrates.
- Then track your weight over the duration of two weeks and adjust if needed.
If your goal is to lose weight, and your weight is not changing, drop calories by another 200 and see what happens over the next two weeks.
If your goal is to gain weight, and your weight is not changing, increase calories by another 200 per day and see what happens over the next two weeks.
Then repeat the process as needed.
So, is Fitbit calorie burn accurate?
Nope – not at all – which is why we recommend the above instead.
Make sure to check out our article on Concurrent training