Test of four activity trackers finds substantial variation in their measures of calories burned and distance covered
Why activity trackers deliver mismatched fitness data — Fitness trackers can monitor our activity 24/7 but how accurate are the devices?
Context & Ripple Effects
The BBC's four-device test lands mid-scrutiny of wearable accuracy: days earlier, a review found Fitbit's new Charge HR and Surge heart-rate sensors disappointing, and the same week a study argued phones track fitness about as well as dedicated wearables — so the premium case for a separate device was already wobbling.
By extending doubt from heart rate to calories burned and distance covered, the test targets the two headline metrics every tracker advertises. The concern proved durable: Fitbit's sensors were later retested by Consumer Reports after a class-action lawsuit, and by 2020 researchers were documenting how darker skin or a shuffling gait can throw off these sensors.
First-order effects
- Shoppers comparing the four devices get materially different calorie and distance readings for identical activity, undercutting the cross-device comparisons and step-count leaderboards that drive tracker purchases.
- Fitbit enters the test already on the defensive from the heart-rate findings, so any wide variance in its numbers compounds a credibility problem with its newest flagship products.
Second-order effects
- If a free phone app tracks activity comparably, the accuracy gap hands consumers a reason to skip the wearable purchase entirely, pressuring device makers to justify their hardware premium on metrics they cannot yet measure reliably.
- Inconsistent measurements create legal exposure: the path from critical reviews to a class-action lawsuit over Fitbit's sensors shows accuracy claims becoming litigation grounds, forcing vendors into third-party retesting.
Third-order effects
- Accuracy shifts from assumed baseline to competitive differentiator, making independent lab testing and published validation a de facto requirement for selling consumer health hardware.
- Documented sensor biases tied to skin tone and gait point toward regulatory or standards-body involvement in consumer health-device claims, since 'wellness estimates' are drifting into territory where systematic error affects some users far more than others.
The trend: Wearables are being forced from feature-led marketing toward independently verified measurement accuracy as reviews, lawsuits, and demographic-bias research converge on the gap between advertised and delivered data.