There are several PREN formulas. They commonly range from: 
: PREN = %Cr + 3.3 × %Mo + 16 × %N 
to:
: PREN = %Cr + 3.3 × %Mo + 30 × %N.  
There are a few stainless steels which add tungsten (W), for those the following formula is used: 
: PREN = %Cr + 3.3 × (%Mo + 0.5 × %W ) + 16 × %N
All % values of elements must be expressed by mass, or weight (wt. %), and not by volume. Tolerance on element measurements could be ignored as the PREN value is indicative only.