An often used approximation of relative permeability is the Corey correlation
which is a power law in saturation. The Corey correlations of the relative permeability for oil and water are then
If the permeability basis is normal oil with irreducible water present, then .
The empirical parameters  and  are called curve shape parameters or simply shape parameters, and they can be obtained from measured data either by analytical interpretation of measured data, or by optimization using a core flow numerical simulator to match the experiment (often called history matching).  is sometimes appropriate. The physical properties  and  are obtained either before or together with the optimizing of  and .
In case of gas-water system or gas-oil system there are Corey correlations similar to the oil-water relative permeabilities correlations shown above.