Purpose: This study aimed to determine the normative reference values of the Evans Index (EI) using magnetic resonance imaging (MRI) and to evaluate its relationship with sex and age. Materials and Methods: This retrospective study reviewed brain MRI images reported as normal from a multispecialty hospital in Southwestern Nigeria, acquired between August 2024 and February 2025. A total of 320 apparently healthy individuals aged 2–83 years were included in this study. The maximum anterior horn width (MAHW) and maximum intracranial diameter (MICD) were measured using electronic calipers on the hospital’s picture archiving and communication system workstation, and the EI was then calculated. All measurements were performed by a single observer. Statistical analyses were performed using IBM SPSS Statistics for Windows, version 23.0. Results: The study included 172 females (53.75%) and 148 males (46.25%), with an average age of 33.07 ± 17.33 years. The mean values for the MAHW, MICD and EI were 31.63 ± 2.72 mm, 127.60 ± 5.80 mm, and 0.25 ± 0.02, respectively. Weak and statistically non-significant correlations were observed between age and the various parameters:MAHW (p = 0.153), MICD (p = 0.267), and EI (p = 0.337). However, there was a small to moderate, statistically significant difference between the sexes, with males showing higher values for all parameters: MAHW [t(292.78) = 5.60, p < 0.001, mean diff. = 1.644], MICD [t(318) = 5.41, p < 0.001, mean diff. = 3.373], and EI [t(318) = 3.22, p = 0.001, mean diff. = 0.0063]. The intraclass correlation coefficients for all measurements demonstrated excellent reliability, with values exceeding 0.90. Conclusion This study established normative EI values for a Southwestern Nigerian population using MRI. These normative values are clinically important, as they assist in the diagnosis of hydrocephalus and neurodegenerative disorders.