Scaling of soil hydraulic properties is a convenient way to characterize soil variability in a unified manner. Scaling techniques implicitly assume geometric similitude for the soils being scaled; however, the meaning of similar media is somewhat ambiguous. This study focuses on the question of how to define geometric similarity. This question is addressed using the physically based scaling (PBS) technique proposed by Kosugi and Hopmans to coalesce 247 soil water retention curves (WRCs) measured from soil cores collected across eight different counties in Indiana, USA. These soil cores represented 29 different soil series and included seven broad textural classes. Although all the 247 WRCs could be scaled together, the RMSEs of the scaling results were improved when WRCs within each textural group were scaled separately. Thus, soil texture may be used as a preliminary guide to group similar soils. This study shows that the sample standard deviations of the pore-size distribution should be used to quantify geometric similarity among soils. Specifically, the coefficient of variation among standard deviations (CVσ) for selected WRCs may be used to demarcate the limit of similarity among soils. Our results show that the CVσ ≤ 10% may be used as a working definition for similarity in soils.