This paper investigates the impact of the waveguide width tolerance in TE 10 mode waveguide TRL/LRL calibration kits. This is important for vector network analyzer measurements in the THZ range where waveguide tolerances become large compared the wavelength and to cross sectional dimensions. Besides causing reflections in the waveguide interface, the waveguide width tolerance also causes a change in the propagation constant that can shift the reference planes and cause problems in estimating the propagation constant of the Line standard. We conclude that the tolerances may cause a significant uncertainty contribution and may limit the useful band of the calibration kit.