The Tandon–Weng model has been used for polymer nanocomposites in previous reports. However, some complex parameters and questionable accuracy of this model have limited its application. In this work, the original Tandon–Weng models for bulk and shear moduli of isotropic composites containing 3D randomly oriented particles are simplified for polymer/clay nanocomposites. Additionally, the effects of main parameters on the predictions of the simplified models are evaluated. According to the simplified models, the moduli of polymer/clay nanocomposites depend to moduli of polymer matrix and nanoparticles, Poisson ratio of matrix, and nanofiller concentration, while some parameters such as aspect ratio of platelets cannot play a significant role. Also, the similar effects of matrix Poisson ratio and nanofiller concentration on the shear and Young’s moduli are observed, but a different trend is fund for bulk modulus.