In Rockwell testing, hardness is determined by measuring the comparative depth of two carefully controlled indentations, one superimposed over the other.

First a Minor Load (one of a lesser force) is applied to a steel ball or sphero—conical diamond penetrator. Then, while the Minor Load is still being applied, a Major Load (one of greater force) is applied at a precisely controlled rate. The Major Load is removed and the hardness determination is displayed.

The hardness determination obtained represents the additional depth to which the Major Load has indented the test specimen beyond the initial indentation of the application of the Minor Load. The displayed indication is proportionally inverse, e.g. the higher the displayed determination of hardness, the shallower the indentations, therefore the harder the material.

In the regular (R) Rockwell scale, the Minor Load is is 10 kgf and the Major Load may be 60, 100, or 150 kgf. In the superficial scale (S), the Minor Load is 3 kgf and the Major Load may be 15, 30, or 45 kgf.

To ensure accuracy and consistency, the Major Load is under precise control and the penetrators are manufactured to specific tolerances. Each point of hardness on the regular (R) scale is 0.00008 inch and 0.00004 inch on the superficial (S) scale, making the need for precise control of the applied forces and precision manufacture of the penetrators readily apparent.