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Background Flow diverter stents (FDSs) are widely used to treat aneurysms in the clinic. However, even the same flow diverter (FD) use on different patients’ aneurysm sites can cause unexpected hemodynamics at the aneurysm region yielding low success rates for the overall treatment. Therefore, the present study aims to unfold why FDs do not work as they are supposed to for some patients and propose empirical correlation along with a contingency table analysis to estimate the flow stasis zones in the aneurysm sacs. Methods The present work numerically evaluated the use of FRED4518 FDS on six patients’ intracranial aneurysms based on patient-specific aneurysm geometries. Computational fluid dynamics (CFD) simulation results were further processed to identify the time evolution of weightless blood particles for six patients’ aneurysms. Results Stagnation zone formation, incoming and outgoing blood flow at the aneurysm neck, and statistical analysis of six patients indicated that FRED4518 showed a large flow stasis zone for an aspect ratio larger than 0.75. However, FRED4518, used for aneurysms with an aspect ratio of less than 0.65, caused small stagnant flow zones based on the number of blood particles that stayed in the aneurysm sac. Conclusion A patient-specific empirical equation is derived considering aneurysms’ morphological characteristics to determine the amount of stagnated fluid flow zones and magnitude of the mean aneurysm velocity in the aneurysm sac for FRED4518 based on weightless fluid particle results for the first time in the literature. As a result, numerical simulation results and patient data-driven equation can help perceive stagnated fluid zone amount before FRED4518 placement by shedding light on neuro-interventional surgeons and radiologists.