The superconformal index (SCI) I(q) of a superconformal field theory (SCFT) is a series of q whose coefficients have information of the operator spectrum of the SCFT. Although there is a formula to calculate the SCI exactly for Lagrangian theories, we need to rely on some non-perturbative methods, like dualities, to obtain the SCI of non-Lagrangian theories. In this talk we discuss calculation of SCI with the AdS/CFT correspondence. It is commonly believed that for finite N field theories it is practically impossible to perform exact calculation on the AdS side due to quantum gravity corrections. I will point out that this is not the case for the SCI and propose a new method to calculate the exact SCI on the AdS side. As an application I calculate the SCI of S-fold theories, which are examples of non-Lagrangian SCFTs.