Building infrastructure “performance” is a term which describes the in-service functioning of a building for a specified use. The concept refers to how effectively, safely, and efficiently a building supports its mission at any time during its life cycle. A building’s performance state, which changes during time in service, is reflected by two different indicators: the physical condition state and the functionality state. The physical condition state relates to a facility’s general “physical fitness,” independent of its mission, as it deteriorates due to routine aging, excessive or abusive use, or poor maintenance. The functionality state relates to the facility’s suitability to function as intended and required for the mission. The functionality state is distinct and independent from the physical condition state. Condition-based metrics are currently in use but a companion index of functionality was not developed at the same time. However, in order to fully describe a building’s fitness for changing missions over its entire life cycle, a quantitative and objective functionality index (FI) is needed. This paper describes the methodology of this FI measurement.