HPFRC/UHPC are today widely applied as repair and reinforcement materials for structures. One of the drawback of commercial HPFRC/UHPC is that any modification of the matrix-fibers mix design, due to the market requirements, affects the mechanical properties of the hardened concrete and therefore the data obtained testing the previous version of the concrete are useless. Prediction models that link the properties of both fibers and matrix to the performances of the hardened concrete are of great practical interest. This paper present a simple and effective model to account for the effect of fibers on the compressive and tensile strengths of HPFRCs as the aspect ratio and volume of fibers changes, extending the range of application of the few existing models.