Pore size in reservoir rocks is measured either directly in thin sections under a petrographic microscope or, nominally, by injecting mercury under pressure into the rock sample. The frequency distribution for nominal pore-size data shows a log-normal pattern similar to that for grain size in sedimentary rocks. Therefore, the same transformed size unit (phi = -log₂ X) traditionally used for grain size can be used for pore-size measurement. A routine was created which measured the entry mercury capillary pressure (Pc_atm), converted this pressure to pore size in millimeters (X_mm = 0.007/Pc_atm), transformed the size values into phi units, and displayed the data as a pore-size frequency distribution. For clean, well-sorted sandstone , the distribution tend to a "bell-shaped" normal curve. Sandstones free from gravel and clay tend to show bimodal pore-size distributions as the grain-size sorting gets poorer. In fine to medium sandstones, a pore-size mode of 5-6 phi is associated with intergranular voids. Authigenic kaolin develops a pore-size mode around 8 phi. Quartz cement tends to obliterate small pores. Carbonate cement does not show preference for pore size, blocking eventually all the pore space. Clay sandstones have a pore-size mode of 11-12 phi. Pores smaller than 2 µm (phi > 9) are considered to be micropores; they provide a large available surface and, accordingly, a high irreducible water saturation.