Darcy's law

Diagram showing definitions and directions for Darcy's law.
From Wikipedia: http://en.wikipedia.org/wiki/File:Darcy%27s_Law.png.

Q - volumetric flow rate (m3/s or ft3/s)
k - permeability, (mDarcy or m2)
A - flow area perpendicular to L (m2 or ft2)
μ - viscosity, (Pa·s)
(Pb - Pa) - pressure drop (Pa or psi)
L - flow path length (m or ft),