Fitness Tracker Accuracy Calculator

Weight training has the highest baseline step error (22%) because gripping a bar or machine handle produces erratic wrist motion instead of the regular step-like swing walking creates, so the accelerometer can't reliably distinguish reps from noise. It also carries the largest heart rate motion penalty (5.0 percentage points added to the MAPE) because gripping and flexing shake the sensor against the skin, disrupting the optical signal the PPG sensor depends on.

It helps up to a point, then stops mattering. The calculator scales error by a factor of (2 minus a sampling-rate ratio capped at 1), so once you reach 50 Hz the ratio hits its ceiling and error gets no further correction — sampling at 100 Hz produces the same estimated accuracy as 50 Hz. Below 50 Hz, though, the penalty grows: at 25 Hz the error multiplier is 1.5x instead of 1x, meaning slower sampling measurably blurs short, jerky motions together.

Skin tone only enters the heart rate model, adding 0.8 percentage points of error per Fitzpatrick step above type 1, because darker skin absorbs more of the green LED light that optical PPG sensors shine through the skin to detect blood flow. Step counting uses the accelerometer, which measures motion mechanically rather than optically, so melanin density has no bearing on it in this model.

Steps Reported is the upper end of the plausible error range — actual steps plus half the estimated step error — while Steps Reported Mid is a deterministic midpoint calculated directly from the step error percentage, without any randomness. Both represent what your tracker might display given the same true step count; the range communicates uncertainty honestly rather than presenting one arbitrary number as if it were exact.