This research aims at quantifying the leakage occurring in Under Floor Air Distribution (UFAD) systems and Over Head Air Distribution (OHAD) systems. The study also classifies the leakage occurring in the different systems into Category 1 leakage and Category 2 leakage. The study was performed between a plenum supply UFAD system (having a maximum flow rate of 1272 m3/h), a ducted supply UFAD system (having a maximum flow rate of 286 m3/h) and an OHAD system (having a maximum flow rate of 2004 m3/h). Results show that in plenum supply UFAD systems, 5.9% and 14.8% of the volume of air supplied by the Air Handling Unit (AHU) is attributed to leakage when the AHU is running at 50% and 100% fan speeds respectively. In ducted supply UFAD systems however, the leakage drops to 2.1% and 5.6% respectively at 50% and 100% AHU fan speeds. A ducted supply UFAD system in comparison to a plenum supply UFAD system lowers leakage by 62.2% when the AHU is operating at 100% fan speed and by 64.4% when the AHU is operating at 50% fan speed.