Objectives We validate a novel CT coronary angiography (CCTA) coronary calcium scoring system. Methods Calcium was quantified on CCTA images using a new patient-specific attenuation threshold: mean + 2SD of intra-coronary contrast density (HU). Using 335 patient data sets a conversion factor (CF) for predicting CACS from CCTA scores (CCTAS) was derived and validated in a separate cohort ( n  = 168). Bland–Altman analysis and weighted kappa for MESA centiles and Agatston risk groupings were calculated. Results Multivariable linear regression yielded a CF: CACS = (1.185 × CCTAS) + (0.002 × CCTAS × attenuation threshold). When applied to CCTA data sets there was excellent correlation ( r  = 0.95; p  < 0.0001) and agreement (mean difference −10.4 95 % limits of agreement −258.9 to 238.1) with traditional calcium scores. Agreement was better for calcium scores below 500; however, MESA percentile agreement was better for high risk patients. Risk stratification was excellent (Agatston groups k  = 0.88 and MESA centiles k  = 0.91). Eliminating the dedicated CACS scan decreased patient radiation exposure by approximately one-third. Conclusion CCTA calcium scores can accurately predict CACS using a simple, individualized, semiautomated approach reducing acquisition time and radiation exposure when evaluating patients for CAD. This method is not affected by the ROI location, imaging protocol, or tube voltage strengthening its clinical applicability. Key Points • Coronary calcium scores can be reliably determined on contrast - enhanced cardiac CT • This score can accurately risk stratify patients • Elimination of a dedicated calcium scan reduces patient radiation by a third