Conductive fillers are often added to thermoplastic polymers to increase the resulting composite's electrical conductivity (EC) which would enable them to be used in electrostatic dissipative and semiconductive applications. The resulting composite also exhibits increased tensile modulus. The filler aspect ratio plays an important role in modeling composite EC, and tensile modulus. It is difficult to measure the filler aspect ratio after the manufacturing process (often extrusion followed by injection molding) in the composite, especially when nanomaterials are used. The EC percolation threshold is a function of the filler aspect ratio; hence, knowledge of this percolation threshold provides a means to extract the filler aspect ratio. In this study, the percolation threshold of the composite was determined from EC measurements and modeling, which in turn was used to determine the filler aspect ratio for tensile modulus modeling. Per the authors' knowledge, this approach has not been previously reported in the open literature. The fillers; carbon black (CB: 210 wt %), multiwalled carbon nanotubes (CNT: 0.58 wt %), or exfoliated graphite nanoplatelets (GNP: 212 wt %); were added to polycarbonate (PC) and the resulting composites were tested for EC and tensile modulus. With the filler aspect ratio determined from EC values for CNT/PC and GNP/PC composites, the three-dimensional randomly oriented fiber Halpin-Tsai model accurately estimates the tensile modulus for the CNT/PC composites and the Nielsen model predicts the tensile modulus well for the CB/PC and GNP/PC composites. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012