Energy consumption is a key concern in the lifetime of a data center. Current efforts to reduce energy consumption focus on the operation of the IT equipment and cooling systems and include expansion of the operating temperature ranges for IT equipment and the raising of cooling system setpoints. Before these can be initiated, however, it is important to ensure data center airflow is properly managed. Much energy is wasted when air from CRAH (computer room air-handling) units bypasses the IT equipment, returning directly to the air handler without cooling any equipment. This wastage of supplied air means that servers draw hot exhaust air to fulfill their required airflow volumes. As this results in higher IT inlet temperatures, CRAH setpoints are often reduced to compensate, leading to higher energy consumption and inefficiency. Appropriate air management minimizes bypass and recirculation of air from server exhausts. Through continued monitoring of airflow, operators are able to target areas for improvement and reduce energy consumption and risk. This paper presents metrics for the air performance of data centers and introduces the concept of air segregation efficiency (ASE), a metric independent of IT and CRAH loads that looks at how well air is segregated in the data center.