Porous pavement is widely used to enhance wet weather skid resistance on highways through its superior drainage capacity. Porosity within the porous pavement structure typically serves as the control variable during the design and construction phases, while permeability is the functional parameter indicating drainage performance during the operation phase. One potential implication of using porosity during design and construction and permeability during operations is that porous pavement layer pore structures resulting from different aggregate sizes can result in different permeability values despite having identical porosity values. Recognizing this issue, this paper first presents a theoretical analysis on the drainage capacity of porous pavement. Outflow tests on typical porous pavements with various aggregate sizes are then simulated using the finite volume method and simulation results are validated against experimental measurements. It is concluded from our study that aggregate size has a significant influence on the permeability of porous pavement and should be included during pavement design, construction and operations.