To study the role of entrance channel parameters on compound nucleus formation probability (Pcn), we have picked up 92 heavy ion fusion reactions forming superheavy nuclei and classified them into four categories based on deformation parameters of projectile and target nuclei such as spherical-spherical, spherical-deformed, deformed-spherical, and deformed-deformed. Next, we have examined the role of entrance channel parameters on Pcn for each category and found that the mean fissility parameter plays the strongest role for all the categories. Finally, the effect of the projectile energy in the centre of mass frame has been taken into account to construct four empirical formulae for the four categories. We believe the proposed empirical formulae can be very useful in the planning of experiments in synthesizing the superheavy elements.