The addition of fibres to concrete can significantly improve the hardened properties but may adversely affect the fresh properties. Apart from entangling with each other to hinder the flow of fresh concrete, the fibres might also interfere with the packing of the solid particles in the concrete. However, the packing density of fibre reinforced concrete made with flexible fibres was rarely measured and thus the effects of flexible fibres on the packing density are still not well understood. Herein, the packing densities of mortar containing polypropylene, carbon or basalt fibres under dry and wet and with and without superplasticizer added conditions were measured and compared to evaluate the effects of the flexible fibres on the packing density. It was found that the packing density of mortar containing these fibres is much higher under wet condition than under dry condition and that the fibre content, fibre length and superplasticizer all have certain influences on the packing density. Based on the test results, best-fit equations revealing the variations of the packing density with the fibre factor under different conditions are derived. (C) 2018 Elsevier B.V. All rights reserved.