  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 from 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 $(5.18\pm0.01) \cdot 10^{-17}$T$\cdot$m$^2$
and $(5.34\pm0.02) \cdot 10^{-17}$ 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.
