We have developed a rigorous and cost efficient method to estimate the onset of entropy along the reaction coordinate in barrierless reactions. Our methodology is based on standard, static computational chemistry, making use of the rigid rotor harmonic oscillator model to estimate entropic contributions. Assuming that upon bond cleavage, indicated by a quantum chemical descriptor, 50% of the total entropy is obtained, we can unambiguously fit a sigmoid function to model the change in along the reaction coordinate. Our methodology only requires the expensive calculation of 2nd order derivatives for the complex and the two separated fragments, while the calculation of the quantum chemical descriptor along the reaction coordinate is computationally inexpensive. Comparison with experimental and computational reference data, showed very good agreement, highlighting the accuracy of our methodology. Investigation of two inorganic complexes showed the applicability to bigger, more complex systems and renders our methodology a cost-effective and universally applicable alternative to more intricate methods, such as VTST, for localisation of transition states in barrierless reactions.