THE tortuosity, T, of a porous medium such as sandstone is a quantity that has been interpreted in various and sometimes conflicting ways, particularly its relation to porosity, ϕ, and formation factor, F (refs. 1–4). Tortuosity originally was understood to mean the square of the quotient average length of a flow-line divided by the net distance traversed; and formation factor was the electrical resistivity of a porous medium divided by the resistivity of the saturating liquid.