Internal instability occurs when the finer fraction from a well-graded granular soil escapes with the infiltrates, rendering a filter ineffective. Thus far, numerous particle size as well as constriction size distribution–based geometrical methods have been proposed to assess potential internal instability. This paper reports the results from hydraulic tests performed on six granular soils (five well-graded sand-gravel mixtures and medium sand) at different uniformity coefficients and compacted at varying relative density. The study facilitated an objective evaluation of some of the well known published methods, leading to a more realistic interpretation of filtration data based on a revised technique, which accurately demarcates the boundary between internally stable and unstable granular soils. A large body of published data and the current laboratory results were used to validate the proposed criterion for the assessment of internal instability, which is also sensitive to the relative density of the filter that has been ignored in most previous methods.