The ratio of the resistivity of a rock filled with water (R_{o}) to the resistivity of that water (R_{w}). G.E. Archie postulated that the formation factor (F) was a constant independent of R_{w} and solely a function of pore geometry (the Archie equation I). It has since been shown that F is independent of R_{w} only for a certain class of petrophysically simple rocks (Archie rocks). In rocks with conductive minerals, such as shaly sands, there is a more complex dependence. In such cases, the ratio R_{o}/R_{w} is known as the apparent formation factor and may vary with R_{w} , temperature and the type of ion in solution. The intrinsic formation factor is then defined as F corrected for the effect of shale, or else the value of R_{o}/R_{w} at the limit of high salinity (low R_{w} ). The correction for the effect of shale depends on the saturation equation used, for example Waxman-Smits, dual water, SGS or CRMM. Unless otherwise stated, the term formation factor usually refers to the apparent formation factor.
F has been related to porosity (phi) by several formulae (Archie, Humble and others) that have the general expression F = a / phi^{m}, where a is a constant and m the porosity exponent.