Compressive stress is computed by dividing the external force by the:

COMPRESSIVE STRESS When a body is placed under a load that tends to compress or shorten it, the internal resistance to such a load is called a compressive stress. A compressive stress is associated with a compressive  strain. To calculate compressive stress, the applied force is divided by the cross-sectional area perpendicular to the axis of the applied force. Key Concept: To calculate compressive stress, the applied force is divided by the cross-sectional area perpendicular to the axis of the applied force. Phillips dental materials 12th edition page no 51