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My Fitbit Air test revealed the flaws of calorie counting with a health tracker

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My Fitbit Air test revealed the flaws of calorie counting with a health tracker - here's why
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ZDNET's key takeaways
- I tested the Fitbit Air's heart rate data.
- I compared it against the Polar H10 chest strap, a reliable heart rate monitor.
- The Fitbit Air is good for heart rate checks, but not as accurate as a calorie counter.
Just how accurate is that health tracker around your wrist? The truth is, you don't really know. The companies that create these devices equip them with sensors that detect every heart rate spike or dip, but their trackers are designed for general, recreational use. Inaccuracies happen.
Still, many of the top health trackers out there are surprisingly accurate, and technological advances have brought each new generation a little closer to the gold standard of exercise and heart rate monitoring.
Also: Google's Fitbit Air is a $99 screenless wearable that I can actually take seriously
I tested this out on the Fitbit Air, one of the fitness world's bigger releases of the year. Using the Polar H10 chest strap as a control, I recorded my gym workout to see how the devices compared. While the Polar H10 chest strap is also a consumer product, it's often considered the gold standard of heart rate monitoring for its technical accuracy. One study found that the chest strap showed "almost perfect agreement" with a reference Lead II ECG system, for example.
My workout routine included both a strength training and treadmill session, my usual blend of weight training and cardio. Here's the breakdown.
On the treadmill
I started out walking, checking the in-the-moment heart rate measurements on both the Polar and the Google Health app. Toggling between the two, the Fitbit Air was within one or two points of the Polar chest strap.
As I started to run, the Polar immediately tracked this increase in heart rate as the Fitbit Air's readings lagged behind. For example, as I began running, Polar captured a heart rate of 141. In that same minute, the Fitbit Air recorded a heart rate of 109. The next minute, Polar recorded 128 bpm, while Fitbit recorded 112 bpm.
Also: I tracked 3,000 steps on my Apple Watch, Google Pixel, and Oura Ring - this one was most accurate
After some calibration, the Fitbit Air caught up to my Polar chest strap's heart rate readings. The devices displayed the same heart rate data while keeping a steady pace, but increasing or decreasing my speed resulted in some momentary discrepancies in the data.
This makes sense, as the Polar chest strap is closer to the heart than the Fitbit and uses electrodes to measure activity and heart rate, so those changes can be quickly recorded, while the Fitbit Air has to wait for that heart rate change to make its way from the heart to the wrist, creating a lapse in data reporting time.
Here are the two heart rate graphs, showing comparable metrics but the slightly different timing, as mentioned above.
| Polar H10 Chest Strap | Fitbit Air | Absolute Difference | % Difference | |
| Calories burned | 143 kcal | 126 kcal | -17 kcal | -11.90% |
| Avg. heart rate | 124 bpm | 122 bpm | -2 bpm | -1.60% |
| Min. heart rate | 96 bpm | 98 bpm | 2 bpm | 2.10% |
| Max. heart rate | 151 bpm | 150 bpm | -1 bpm | -0.70% |
The heart rate metrics performed swimmingly with under 2.5% error. The Fitbit Air underestimated calories burned by nearly 12% during my treadmill session. These trackers calculate calories burned through a formula that factors in heart rate, weight, age, sex, activity intensity, and more. It's not as simple as gathering heart rate data, so errors can compound, as we will continue to see in my strength training data.
Strength training
To compare Fitbit Air's accuracy to the H10 chest strap, I used both devices to track my full body workout at the gym. This consisted of lat pulldowns, dumbbell squats to shoulder presses, dumbbell lateral raises, stability ball exercises, and the hip abductor machine.
| Polar H10 Chest Strap | Fitbit Air | Absolute Difference | % Difference | |
| Calories burned | 152 kcal | 105 kcal | - 47 kcal | -30.90% |
| Avg. heart rate | 100 bpm | 101 bpm | 1 bpm | 1% |
| Min. heart rate | 66 bpm | 67 bpm | 1 bpm | 1.50% |
| Max. heart rate | 151 bpm | 134 bpm | -17 bpm | -11.3 |
In both exercises, the minimum and average heart rate datapoints are the most accurate. Calories burned and max heart rate have a greater deviation from the chest strap, with calories burned tripling its inaccuracy from my treadmill session.
Also: Whoop vs. Fitbit Air: I used both to track my health and fitness for a month - this one's better
Unlike in the first test, the maximum heart rate is off by 11% in this test. This could indicate that the Fitbit Air is missing brief spikes in heart rate during shorter bursts of intensity, as my heart rate rose and fell in between my strength training exercises and breaks.
Because my strength training sessions involved a few short bursts of higher intensity and then dips back to a baseline, the Fitbit Air may have missed the peaks of these max heart rate spikes. The second test proves that the maximum heart rate is more unreliable in weight training sessions than it is in steady sessions, like a treadmill or cardio exercise.
The bottom line
I'll start out by disclaiming that two tests is not enough to draw strong conclusions on the overall accuracy of the Fitbit Air. This was an amateur test at my local gym -- not a trial in a lab. However, this test has helped me understand health trackers in a new way.
For example, wrist or finger-worn health trackers are not going to capture the quickest fluctuations in heart rate, because of their position on the body. They're farther away from the heart, so it takes a little longer for blood to pump to these extremities and reflect the most momentary heart rate fluctuations.
This results in a device that won't always capture the subtlest changes in heart rate or reflect them in your workout stats. Opt for a heart rate monitor if that's something you value during training.
Also: I wore the Oura Ring 5 for a month, and two big upgrades make it so much better than the 4
The Fitbit Air ended up recording mostly accurate heart rate data that matched the chest strap's graphs overall. Despite the strength training's maximum heart rate data's 11% deviation from the Polar chest strap, the device recorded heart rate data with low inaccuracy. I was very impressed by these datapoints and would recommend the Fitbit Air as a reliable health tracker for these very reasons.
But for calorie tracking... that's another story. It's difficult already to calculate calories burned, and each tracker will calculate it differently. I didn't expect the Fitbit Air to be 11% to 30% off in its calories burned estimations, however. If you are using the device for diet or weight management, I'd recommend taking those datapoints as ballpark estimations rather than objective points.

Facts Only

* The Fitbit Air heart rate data was tested against the Polar H10 chest strap.
* The Fitbit Air lagged behind the Polar chest strap during a treadmill run.
* During one minute of running, the Polar recorded 141 bpm while the Fitbit recorded 109 bpm.
* When tracking a steady pace, heart rate metrics performed with under 2.5% error between the devices.
* In strength training, calorie burned differed by 47 kcal (30.90%) between the Polar H10 and the Fitbit Air.
* Maximum heart rate data showed a 11.3% deviation in strength training sessions.
* Minimum heart rate data showed a 1.50% deviation in strength training sessions.
* Calorie calculation relies on a formula factoring in heart rate, weight, age, sex, and activity intensity.

Executive Summary

Testing the Fitbit Air against a more established heart rate monitor, the Polar H10 chest strap, revealed discrepancies when tracking both heart rate and calorie expenditure during different physical activities. When comparing heart rate data during a treadmill session, the Fitbit Air lagged behind the Polar chest strap by several minutes, indicating a delay in reporting physiological changes from the wrist compared to the chest strap's direct measurement. In strength training, while average and minimum heart rates were closely aligned (within 1% to 1.5%), the maximum heart rate readings showed greater divergence, with the Fitbit Air showing an 11.3% difference compared to the chest strap for a session including weight training and cardio.
The analysis suggests that wrist-worn trackers may not capture the quickest fluctuations in heart rate due to their physical position relative to the heart, which is more accurately measured by chest straps. Furthermore, calorie tracking proved significantly less consistent; during treadmill use, the Fitbit Air underestimated calories burned by approximately 12% when compared to the Polar H10. The author concludes that while the Fitbit Air provided generally reliable heart rate data, users should view calorie estimations from these devices as ballpark figures rather than objective measurements for diet or weight management.

Full Take

The comparison between the wrist-worn tracker and the chest strap highlights a fundamental conflict between temporal fidelity and physiological measurement location. The disparity in heart rate reporting timing suggests that peripheral sensors face latency issues when capturing rapid cardiac fluctuations compared to direct measurements taken closer to the source. This points to an architectural limitation: accuracy in specific metrics (like sustained heart rate) can be achieved via multiple methods, but synchronizing them across different sensor modalities introduces unavoidable error margins, particularly during dynamic changes. The failure in calorie tracking demonstrates that complexity—where energy expenditure relies on integrated physiological and physical data—compounds errors far more significantly than simple heart rate measurements alone. This pattern suggests that devices designed for convenience often prioritize a streamlined output over the granular accuracy required for objective measurement, especially when those measurements are subject to compounding variables like body composition and movement intensity. The implication is that for high-stakes applications such as precise fitness tracking or caloric management, relying on aggregate estimations from peripheral sensors introduces systemic risk, demanding a shift in user expectation toward using dedicated monitors when accuracy is paramount.

Sentinel — Human

Confidence

LIKELY_HUMAN (confidence: 0.2)

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