Provided herein are embodiments of a method and apparatus for measuring permeability of a rock core sample in the direction of pressure gradient. In one embodiment, a method includes providing a rock core sample having an inlet primary face, an outlet primary face and one or more sidewalls having a thickness (T) adjoining the inlet and outlet primary face, wherein the inlet primary face has a surface area (SAIF) and the one or more sidewalls have a surface area (SASW), and further wherein a ratio of the inlet primary face surface area (SAIF) to sidewall surface area (SASW) is at least 0.4; placing the rock core sample, unbounded, between an inlet flow plate and an outlet flow plate; directing a fluid through from the inlet flow plate into the rock core sample; and measuring a flow rate of the fluid exiting the outlet flow plate.