The Pitting Resistance Equivalent Number (PREN) is a practical measure for comparing the pitting-corrosion resistance of stainless steels.  The pitting resistance of stainless steel is primarily determined by its composition.  The elements which have a significant impact are chromium, molybdenum, and nitrogen.  The formula, which quantifies their respective contribution, is:


In the literature, the factor for nitrogen varies according to the source.  Some variants of the equation include also tungsten (W).

PREN values are useful for ranking stainless steel grades according to their pitting-corrosion resistance.  However, this comparison is only meaningful within the same stainless steel family (austenitic, ferritic, duplex).  PREN cannot be interpreted as an absolute value.