A test for determining the hardness of metals by applying a known load to the surface of the material to be tested with a hardened steel ball of known diameter. The diameter of the resulting impression in the metal is measured. The Brinell hardness number is calculated as the quotient of the applied load divided by the area of the surface of the impression, which is assumed to be spherical. If P is the applied load (measured in kilograms), D is the diameter of the steel ball
(measured in millimeters) and d is the diameter of the impression (measured in millimeters), then
in which B.H.N. is the Brinell hardness in kilograms per square millimeter. See HARDNESS TESTING.