Okay I found some references and the equation I am after is Q = 2 * C * (D * t / ∏)^(1/2) where Q is the weight of drug released per unit area (hence unit is mg/cm^2) C is the initial drug concentration D is the diffusion coefficient (unit of cm^2 min^-1) and t is release time in min. What I am not sure about is the units. Logically the concentration of drug would be in g/L. but when I merge all the units together, I get something else. This is my working. The units on the right hand side should equals to Q. = 2 (constant) * C (g/L) * [D (cm^2 * min^-1) * t (min) / ∏ (constant)]^(1/2) only considering units (ie discard constants) = [g * cm^[2*(1/2)] * min^(1/2)] / [L * min^(-1 * 1/2)] = g * cm * min^(1/2) / L * min^(-1/2) = g * cm * min^(1/4) / L which does not equals to unit of Q (mg/cm^2). The units given are all correct but is there something I am missing here? Breaking rules of powers perhaps? Thank you!!!!