Flat polyester yarn is bulked by the mechanical crimp texturizing process. The magnitude of the false-twist, introduced together with the torsional rigidity of the supply yarn, has a significant effect on the transformation process. The torsional rigidity of the supply yarn is in turn influenced by its fineness and bending characteristics. It is therefore deemed necessary to establish a mathematical relationship between the optimum false-twist level and the yarn fineness for materials with known bending characteristics. Such a relationship may help in producing a yarn with optimum bulk properties, while minimizing resource utilization and waste generation. This paper reports research work done in this direction. Polyester yarn is chosen for this investigation as it has a wide range of applications. Fully drawn multifilament polyester yarns, with different linear densities, are textured on a single-head mechanical crimp texturing machine. The results are analysed with the help of polynomial curve fitting (Polyfit) using MATLAB. This mathematical tool facilitated the derivation of the desired formula (polynomial equation) for the calculation of optimum false-twist level for fully drawn multifilament polyester yarns undergoing mechanical crimp texturizing.