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
Area of the test specimen upon which the weight rests
Elasticity of the test specimen in strain
Length of the test specimen beneath the force
Strain of the test specimen per unit Length