The drug flux represents the slope of the linear regression of the accumulated mass (Q) over time (t) normalized over the permeation area (A), i.e., the surface area of the barrier available for permeation.
Equation 1: 
The drug apparent permeability (P) is calculated by normalizing the drug flux (j) over the initial concentration of the API in the donor compartment (c) as:
Equation 2: 
Dimensionally, the P represents a velocity, and it is normally expressed in cm/sec. The highest is the permeability, the highest is expected to be the bioavailability of the drug after oral administration.