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A measure of ability of a porous formation to allow a certain fluid to pass through it.

Symbol	Dimension	SI units	Oil metric units	Oil field units
	L²	m²	md	md

For the laminar flow:

k = \mu \cdot \frac{| {\bf v}|}{ | \nabla p |}

where

	fluid viscosity
	fluid velocity
	pressure gradient

Permeability depends on fluid type, filling the porous media and the fluid type which is sweeping through it which leads to splitting its value into a product of two components:

k = k_a \cdot k_r

where

	absolute permeability to air which is defined by the reservoir pore structure only, also denoted as or
	relative permeability to a given fluid which is defined by the interaction between fluid and reservoir matrix

In general case, permeability is anisotropic both in vertical and lateral directions and quantified by symmetric tensor value:

k=\begin{bmatrix}
    k_{11}  & k_{12} & k_{13}   \\
    k_{12}   & k_{22} & k_{23}   \\
    k_{13}    & k_{23} & k_{33} 
\end{bmatrix}

which can be diagonalized for a proper selection of coordinate axis :

k=\begin{bmatrix}
    k_x  & 0 & 0   \\
    0   & k_y & 0   \\
    0    & 0 & k_z 
\end{bmatrix}

and characterized by 3 principal tensor components

If not mentioned otherwise the permeability usually means absolute horizontal permeability: .

See also