For the two possible arrangements of a rectangular waveguide in a magnetic field, small or large side parallel to the field, the radiation induced line broadening in FMR experiments in a rectangular waveguide terminated to both sides is calculated in dependence on the sample position. The theory, which allows the determination of the intrinsic linewidths ΔH0 from the measured linewidths ΔHeff, is compared to experimental investigations made at X‐band on three single crystal YIG‐spheres with various diameters. Additionally a possibility is shown to decide experimentally whether the uniform precession mode is excited in a sample or not or which magnetostatic mode may be excited.