In petroleum engineering, Lak wettability index is a quantitative indicator to measure wettability of rocks from relative permeability data. This index is based on a combination of Craigs first rule. and modified Craigs second rule  
where 
: : Lak wettability index (index values near -1 and 1 represent strongly oil-wet and strongly water-wet rocks, respectively)
: : Water relative permeability measured at residual oil saturation 
: : Water saturation at the intersection point of water and oil relative permeability curves (fraction)
: : Residual oil saturation (in fraction)
: : Irreducible water saturation (in fraction)
: : Reference crossover saturation (in fraction) defined as:
and  and  are two constant coefficients defined as:
:   and          if  
:   and          if  
:   and        if  
To use the above formula, relative permeability is defined as the effective permeability divided by the oil permeability measured at irreducible water saturation.