Background and Purpose: Patient selection for thrombectomy of acute ischemic stroke caused by large vessel occlusion in the delayed time window (>6 hours) is dependent on delineation of clinical-core mismatch or radiological target mismatch using perfusion imaging. Selection paradigms not involving advanced imaging and software processing may reduce time to treatment and broaden eligibility. We aim to develop a conversion factor to approximately determine the volume of hypoperfused tissue using the National Institutes of Health Stroke Scale (NIHSS) score (clinically approximated hypoperfused tissue [CAT] volume) and explore its ability to identify patients eligible for thrombectomy in the late-time window. Methods: We performed a retrospective analysis of anterior circulation large vessel occlusion strokes at 3 comprehensive stroke centers. Demographic, clinical, and imaging (computed tomography perfusion processed using RAPID, IschemaView) information was analyzed. A conversion factor, which is a multiple of the NIHSS score (for NIHSS score <10 and ≥10), was derived from an initial cohort to calculate CAT volumes. Accuracy of CAT-based thrombectomy eligibility criteria (using CAT volume instead of Tmax >6 seconds volume) was tested using DEFUSE-3 criteria (Endovascular Therapy Following Imaging Evaluation for Ischemic Stroke 3) eligibility as a gold standard in an independent cohort. Results: Of the 309 large vessel occlusion strokes (age, 70±14, 46% male, median NIHSS 16 [12–20]) included in this study, 38% of patients arrived beyond 6 hours of time from last known well. Conversion factors derived (derivation cohort-center A: 187) based on median values of Tmax>6 second volume for NIHSS score <10 subgroup was 15 and for NIHSS score ≥10 subgroup was 6. Subsequently calculated CAT volume–based eligibility criteria yielded a sensitivity of 100% and specificity of 92% in detecting DEFUSE-3 eligible patients (area under the curve, 0.92 [95% CI, 0.82–1]) in the validation cohort (center B and C:122). Conclusions: Clinical severity of stroke (NIHSS score) may be used to calculate the volume of hypoperfused tissue during large vessel occlusion stroke. CAT volumes for NIHSS score <10 (using a factor of 15) and ≥10 (using a factor of 6) subgroups can accurately identify DEFUSE-3-eligible patients.