As explained in ASTM E10-18:
“The Brinell hardness test is an indentation hardness test that can provide useful information about metallic materials. This information may correlate to tensile strength, wear resistance, ductility, or other physical characteristics of metallic materials, and may be useful in quality control and selection of materials. Brinell hardness tests are considered satisfactory for acceptance testing of commercial shipments and have been used extensively in industry for this purpose.”
An indenter of known diameter is pressed into a material with a known force load, typically measured in kilogram-force (kgf). The diameter of the indentation after the load is measured, and these values are input into the following formula:

where BHN is the Brinell hardness number, P is the force load in kgf, D is the ball indenter diameter in mm, and d is the indentation diameter in mm. For a given force and indenter size, a larger indentation indicates a softer material, and results in a lower BHN.