The effective mass of charge carriers is a crucial parameter for the design of any optoelectronic device. The estimated values of the effective mass of 2D halide perovskites currently span a broad range, providing an unwelcome source of confusion in this promising material system. Here we highlight how the distortion imposed by the organic spacers, and orbital hybridization effects by the metal cation, govern the effective mass. As a result, the effective mass in 2D halide perovskites can be easily tailored over a wide range. To demonstrate this, we have directly measured the reduced effective mass of charge carriers in phenethylamine (PEA)-based 2D halide perovskites. Combining the experimental results with electronic band-structure calculations, we propose a scaling diagram for the effective mass value versus the distortion of the octahedra imposed by the organic cations.