As per ASTM E18, the indenters can either be diamond spheroconical or tungsten carbide balls. If a diamond spheroconical indenter is used, then Rockwell Hardness Number = 100 - (h/0.002) If a ball indenter is used, then Rockwell Hardness Number = 130 - (h/0.002) Where 'h' is the indentation depth in mm.