Fitbit is the world’s most popular activity tracker, calculating your daily activity and motivating you to move throughout the day. But how accurate is a Fitbit? Find out how Fitbit calculates your steps and how well it tracks your steps, calories burned, and sleep.

Fitbit uses a three-axis accelerometer that can detect movement in any direction. When worn on the body, a patented algorithm that looks for specific movement patterns analyzes the data captured by Fitbit's accelerometer.

The data from the accelerometer and the counting algorithm together determine the number of steps taken, the distance traveled, energy consumption, exercise intensity and sleep.

Experts say Fitbits are surprisingly accurate, but they’re not perfect. Because movement is subject to a variety of factors, they’ve been known to undercount or overcount steps. Walking on soft carpet or pushing a shopping cart or stroller can cause Fitbits to undercount steps. Riding down a bumpy road or riding a bike can cause it to overcount steps.