1. A portable activity monitoring device, adapted to couple to a body of a user, to calculate a number of stairs or flights of stairs traversed by the user, the portable activity monitoring device comprising: a housing having a physical size and shape that is adapted to couple to the body of the user; an altitude sensor, disposed in the housing, to detect a change in altitude of the user and, in response thereto, to generate data which is representative of the change in altitude of the user; and processing circuitry, disposed in the housing and electrically coupled to the altitude sensor, to calculate a number of stairs or flights of stairs traversed by the user using the data which is representative of a change in altitude of the user.