Measurements of the exchange stiffness $D$ and the exchange constant $A$ of Yttrium Iron Garnet (YIG) films are presented. The YIG films with thicknesses from 0.9 $mu$m to 2.6 $mu$m were investigated with a microwave setup in a wide frequency range f
rom 5 to 40 GHz. The measurements were performed when the external static magnetic field was applied in-plane and out-of-plane. The method of Schreiber and Frait, based on the analysis of the perpendicular standing spin wave (PSSW) mode frequency dependence on the applied out-of-plane magnetic field, was used to obtain the exchange stiffness $D$. This method was modified to avoid the influence of internal magnetic fields during the determination of the exchange stiffness. Furthermore, the method was adapted for in-plane measurements as well. The results obtained using all methods are compared and values of $D$ between $(5.18pm0.01) cdot 10^{-17}$T$cdot$m$^2$ and $(5.34pm0.02) cdot 10^{-17}$ T$cdot$m$^2$ were obtained for different thicknesses. From this the exchange constant was calculated to be $A=(3.65 pm 0.38)~$pJ/m.
