The present disclosure provides a system, method, and computer program for conditioning a building environment based on occupancy estimates. In one embodiment, the method includes tracking the user's entries to and exits from the building on a mobile device associated with the user for each of a plurality of users. Entry and exit event data from the users' mobile devices are used to estimate occupancy of the building. The conditioning of the building environment is adjusted in accordance with changes in occupancy estimates. In certain embodiments, where the building has a plurality of zones, each entry and exit event may be correlated with one of the plurality of zones. Occupancy is estimated within each zone using the zone-correlated entry and exit events. Alternately, users' locations within the building may be estimated using an indoor localization technique. Each zone is then conditioned in accordance with the occupancy of the zone.