An inertial device that is integratable in a portable electronic device includes: an inertial sensor for generating at least one raw acceleration signal in response to accelerations caused by movements of walking and running of a user of the pedometer; and a processing unit, associated to the inertial sensor for counting a number of steps of the user of the pedometer on the basis of the raw acceleration signal. The inertial sensor and the processing unit are both encapsulated within a single package for integrated circuits, which can be coupled to a circuit board of an electronic device and is provided with at least one connection terminal for making the number of steps available to the outside world.