In this thesis we investigated magnetic excitations in the iridate compound Sr2IrO4. As a layered strongly correlated system it can be described by the two-dimensional one-band Hubbard model, describing interacting particles in effective spin-1/2 states. Based on the previously calculated band structure model, it becomes an effective model with one free parameter, the one-site interaction strength U. Using Green's functions at the mean field level, we calculated the dynamic magnetic susceptibility and thereby the spin wave dispersion. We compared the spin wave dispersion with resonant inelastic X-ray scattering experiments and used it to fit the Hubbard interaction strength U. We found the t-t'-t''-U-Hubbard model based on the band structure energies to provide a good description of the spin wave dispersion for U=1.1eV.