The primary way a Fitbit works is by using an accelerometer, which senses movement and interprets it as data that counts steps and other exercise. Fitbits use algorithms, a type of mathematical formula or procedure, to translate movement into data.
Fitbits have a three-axis accelerometer that tracks more detailed movements than single-axis models. This allows it to track more subtle or unusual movements than a traditional pedometer, which is how it tracks sleep. Fitbits use algorithms to determine whether a movement is large enough to indicate lighter or disturbed sleep.
Fitbits also track the amount of calories burned, which is based on another mathematical formula. The device uses the wearer's height, weight, age and gender to determine how many calories an activity burns. Some Fitbits also have a heart rate tracker, and that information is used in the calculation as well.
Some Fitbits also count the number of floors climbed. This function works using an altimeter, which uses changes in barometric pressure to determine elevation.
These functions do have room for error. Fitbits may miscount steps, interpreting bouncing during a car ride or other non-walking motions as steps. It may also miss steps if the step is too small or smooth. Similarly, the sleep tracking function starts after the tracker has been motionless for a time, so users who are reading or watching television in bed may get inaccurate sleep data.