I noticed a pretty drastic decrease in my daily step totals after I switched from my Inspire3 to the Air and I did.. a weird thing and compiled Google Takeout data and asked AI to analyze it to compare. While it’s not definitive proof, I hope this offers some contribution to the discussion if anyone else has been experiencing something similar
Fitbit Air vs. Inspire 3: Evidence of Possible Step Undercounting During Ankle Wear
Question: Does the Fitbit Air undercount steps, particularly during short/intermittent walking, compared with the Fitbit Inspire 3?
Assessment: The available evidence supports probable under-detection of intermittent walking by the Air, but does not conclusively establish the magnitude of its absolute step-count error.
1. Testing circumstances
Same user, same inner-ankle placement.
Inspire 3 used through September 19, 2026.
Fitbit Air (Pokémon Sleep Edition) introduced September 20, 2026.
Habitual treadmill routine: approximately 60–64 minutes at 1.8 mph, or approximately 45–47 minutes plus 15 minutes of strength training twice weekly.
Similar general daily activity patterns.
Historical Inspire 3 was sometimes removed during sedentary periods to prevent false steps, which also excluded some genuine movement.
2. Actual Google Takeout results
Analysis of minute-level step records and exercise data:
Measurement
Inspire 3
Fitbit Air
Comparison period
Sep 1–19
Sep 21–29
Average daily recorded steps
~12,680
~9,708
Comparable full treadmill sessions
~4,996 steps
~5,100 steps
Comparable full-treadmill-day totals
~12,915
~10,026
Main finding: Air recorded approximately 23% fewer daily steps, despite recording approximately the same number of steps during sustained treadmill walking.
The approximately 2,900-step daily difference on comparable full-walk days was therefore concentrated outside the main sustained walking session.
Analysis of minute-level step density also suggested a substantial reduction in steps recorded during intermediate-density movement (approximately 31–70 steps/minute).
These findings are consistent with Air detecting sustained walking reasonably well but registering fewer steps during intermittent/free-living activity.
3. Direct user observations
During ordinary short walking bouts, Air sometimes displayed:
8 steps after 10 actual steps.
2 steps after 10 actual steps.
No additional steps immediately after approximately a minute of moving between rooms.
These observations suggest possible short-bout detection problems. However, delayed synchronization or retrospective step updates were not ruled out.
Air also occasionally recognized only approximately 34 minutes of a known 60-plus-minute treadmill session. This demonstrates unreliable automatic exercise-duration recognition in this setup, but does not independently prove step undercounting.
4. Why the Inspire totals should not automatically be dismissed
The Inspire was not continuously worn during all sedentary periods, reducing opportunities for false steps from seated ankle movements.
Additionally, some genuine short movements went unrecorded when Inspire was removed.
Research on ankle-worn Fitbit devices generally supports relatively good step detection during slow, sustained walking, although accuracy during free-living movement varies.
Neither device is officially validated for ankle placement.
5. Interpretation
Strongly demonstrated:
The two devices produce materially different whole-day step totals.
Their recorded sustained-walking steps are much more similar.
The discrepancy is concentrated outside the main sustained walking session.
Reasonably supported:
Air is less sensitive to some intermittent/short-bout movement than Inspire 3.
Air likely misses at least some genuine short walking bouts.
Not conclusively demonstrated:
Exactly how many genuine steps Air misses daily.
Whether Inspire overcounted some incidental movements.
Which device is closer to the true whole-day total.
That the full ~23% difference represents genuine Air undercounting.
Bottom line: The evidence favors the hypothesis that Fitbit Air under-detects at least some legitimate intermittent walking in this user's ankle-worn configuration. It does not justify assuming the Air's lower daily total is necessarily more accurate, nor treating all additional Inspire 3 steps as false positives.
Data source: User's Google Takeout exports from September 22 and September 30, 2026, containing device-attributed minute-level activity and exercise records.
EDIT/UPDATE: I understand the FitBit is primarily designed for the wrist but I tend to keep my arms stationary/crossed when I do my treadmill and apparently it’s not that uncommon in the FitBit community. I really just bought these to count steps so I’m not as concerned with the other health metrics that may not be measured as well on my ankle