Fitbit Air's calorie counting falls short in real-world test

ZDNET's Wearables & Health Tech Editor Nina Raemont put Google's newly released Fitbit Air ($99 screenless wearable) to the test against the Polar H10 chest strap—a device considered the gold standard for heart rate monitoring accuracy.

During mixed gym sessions combining strength training and treadmill runs, Raemont found Fitbit Air's readings routinely lagged behind Polar H10's. While both devices aligned during steady-state cardio, the discrepancies became pronounced during intensity changes. For instance, Polar registered 141 bpm as Raemort transitioned to running, while Fitbit Air recorded just 109 bpm in the same minute—a 32 bpm difference. These gaps narrowed after calibration but persisted during speed adjustments. The test underscores the limitations of wrist-based sensors for precise calorie counting, as even minor heart rate inaccuracies compound into significant calorie estimation errors over time.

The findings highlight a critical flaw in consumer-grade health trackers: while they excel at general activity tracking, their sensors and algorithms aren't optimized for scientific-grade accuracy. Fitbit Air's real-time heart rate checks remain usable for casual users, but professionals relying on precise calorie data (e.g., athletes or data-driven fitness enthusiasts) should supplement wrist-based trackers with dedicated chest straps. The test serves as a reminder that health metrics from wearables should be interpreted as estimates rather than hard data.