Due to the finite aspect ratio of the ferrite, the distribution of the internal field in small planar microwave ferrite devices is strongly inhomogeneous. This is a source of additional losses through magnetostatic modes. To study the magnetostatic modes, the thickness dependence of the ferromagnetic resonance spectra was measured for a series of 2 mm square single crystal yttrium–iron–garnet films in the 35<h<220 μm thickness range. The in-plane resonance field Hr increases with thickness by 150 G, while Hr normal to film plane decreases. The uniform mode linewidth, dHr is in the 2–20 G range. A large number of magnetostatic wave modes modes is excited, covering the “full spectral width,” DH, increasing with thickness from 400 to 960 G. In a ferrite device DH has to be taken into account, giving rise to large losses in spite of the small intrinsic linewidth dHr of the material.