Fitbit trackers use a 3-axis accelerometer to understand your motions. An accelerometer is a device that turns movement (acceleration) of a body into digital measurements (data) when attached to the body. By analyzing acceleration data, our trackers provide detailed information about frequency, duration, intensity, and patterns of movement to determine your steps taken, distance traveled, calories burned, and sleep quality. The 3-axis implementation allows the accelerometer to measure your motion in any way that you move, making its activity measurements more precise than older, single-axis pedometers.
Fitbit trackers have a finely tuned algorithm for step counting. The algorithm is designed to look for motion patterns most indicative of people walking. One condition for a motion pattern to be recognized as a step is the motion must be large enough. The algorithm implements this by setting a threshold. If a motion and its subsequent acceleration measurement data meet the threshold, the motion will be counted as a step. If that threshold is not met, the algorithm won’t count the motion as a step. Other factors can create enough acceleration to meet our threshold and therefore cause some over counting of steps, such as riding on a bumpy road. Equally, it's possible for the algorithm to undercount (not meet the required acceleration threshold). Examples here include walking on a very soft surface such as a plush carpet.