Rubber fracture mechanics can be used to predict fatigue lifetime of an elastomer product, and the essential inputs are the crack growth rate law and the size of crack precursors. A distribution of crack precursor sizes leads to a distribution of failure properties, such as fatigue lifetime and tensile strength. Undispersed filler agglomerates or other large inclusions/contaminants in the rubber can act as crack precursors that limit the strength and lifetime of the material. To demonstrate these relationships, we use tensile strength data from 50 specimens to characterize the failure distribution behavior of each rubber compound and compare the results to filler agglomerate size distribution measured using microscopy. Poor mixing is simulated by adding a portion of the carbon black filler late in the mixing process, and glass microspheres are introduced during milling to reproduce the effects of large inclusions.