The examination of cross metathesis reactions leading to the desired product has been conducted to uncover computationally the origin of the chemo-, regio- and stereoselectivity. The comparison between the relative stabilities of all involved intermediates and products, together with the transition states, links to the probability for the respective pathway. Particularly, the respective transition states for each reaction tune the regio- and stereoselectivity because they lead to define the energy barriers needed to be overcome to form the new olefin as final product. The broad range of studied reactions with the 2nd generation Grubbs catalysts allows concluding in detail the points to pay attention and thus helps to understand the chemo-, regio- and stereoselectivity in new olefin metathesis reactions. Here, a web-server joins all these mechanistic insights which is intended to support future predictive olefin metathesis catalysis.