PURPOSE: To determine the effect of model-based iterative reconstruction (IR) on coronary calcium quantification using different submillisievert CT acquisition protocols. METHODS: Twenty-eight patients received a clinically indicated non contrast-enhanced cardiac CT. After the routine dose acquisition, low-dose acquisitions were performed with 60%, 40% and 20% of the routine dose mAs. Images were reconstructed with filtered back projection (FBP), hybrid IR (HIR) and model-based IR (MIR) and Agatston scores, calcium volumes and calcium mass scores were determined. RESULTS: Effective dose was 0.9, 0.5, 0.4 and 0.2mSv, respectively. At 0.5 and 0.4mSv, differences in Agatston scores with both HIR and MIR compared to FBP at routine dose were small (-0.1 to -2.9%), while at 0.2mSv, differences in Agatston scores of -12.6 to -14.6% occurred. Reclassification of risk category at reduced dose levels was more frequent with MIR (21-25%) than with HIR (18%). CONCLUSIONS: Radiation dose for coronary calcium scoring can be safely reduced to 0.4mSv using both HIR and MIR, while FBP is not feasible at these dose levels due to excessive noise. Further dose reduction can lead to an underestimation in Agatston score and subsequent reclassification to lower risk categories. Mass scores were unaffected by dose reductions.