Existing buildings consume most of our non-transportation related energy and are a major contributor to the emission of greenhouse gases. Improving their energy efficiency by applying energy conservation measures (ECMs) remains a major challenge. This paper presents computational tools developed to screen office buildings for retrofit and to select specific ECMs for each building. This selection process accounts for climate, occupancy, building operation, heating and cooling systems, distribution systems, envelope properties, building geometry, and cost of ECMs. The tools screen the complete building set for optimal retrofit opportunities. The methodology adopted in developing the computational tools includes characterization of the office building stock into a manageable set of archetypes; simulation of building operation using specific occupancy characteristics and local climate data; development of energy prediction models for lights, equipment, pumps, fans, domestic hot water, chillers, and boilers; and application of present value analysis to evaluate the cost effectiveness of ECMs.