Hardness is the resistance to indentation, and it is determined by measuring the permanent depth of the indentation. When using a fixed force (load) and a given indenter, the smaller the indentation, the harder the material.

Indentation hardness value is obtained by measuring the depth or the area of the indentation using one of over 12 different test methods.

The Rockwell hardness test method, as defined in ASTM E-18, is the most commonly used hardness test method.