In this retrospective study, a simplified ordinal scoring method referred to as the modified length-based grading was validated for assessing coronary artery calcium (CAC) severity on non-electrocardiogram (ECG)-gated chest computed tomography (CT). The study included 120 patients who underwent both non-ECG-gated chest CT and ECG-gated cardiac CT. Six radiologists independently used two scoring methods, visual assessment and modified length-based grading, to assess CAC severity on chest CT. The Agatston score categories on cardiac CT served as the reference standard. The results indicated that the modified length-based grading method demonstrated better interobserver agreement and agreement with cardiac CT than visual assessment. In this study, CT images and patient information were reviewed and anonymized using a dedicated server, AVIEW Research, by Coreline Soft Inc., which was accessed by the six observers for image review and grading. This study concludes that the modified length-based grading method effectively evaluates CAC on non-ECG-gated chest CT.