A laboratory investigation has been made of formation factor‐porosity relationships (formation factor being the ratio of the resistivity of a porous medium to the resistivity of the pore‐fluid), using natural and artificial sand samples whose grains varied widely in both size and shape. All samples obeyed Archie’s law, [Formula: see text] (where FF is the formation factor and n is the porosity) including mixtures of two differently shaped particle types. The exponent m was dependent on the shape of the particles, increasing as they became less spherical, while variations in size and spread of sizes appeared to have little effect. The results have been combined to produce an FF/n relationship, with an error “envelope”, which may be applicable to marine sediments in general, being in agreement with published data for marine clays. It is also suggested that the exponent m may be a better measure of the “tortuosity” of porous media than the formulas quoted in the literature.