Background <p>Ionized calcium (iCa) has physiological activity in the body; however, the direct measurement of iCa is technically limited. Although total serum calcium (tCa) and corrected serum calcium (cCa) using the Payne correction formula have been used to assess calcium levels, their limitations have been noted. We aimed to clarify the degree of correlation and dissociation among iCa, tCa, and cCa levels in patients undergoing hemodialysis.</p> Methods <p>This cross-sectional study assessed the correlation between iCa, tCa, and cCa levels in patients undergoing hemodialysis. Factors involved in the correlation between iCa and tCa levels were evaluated using multiple regression analysis. Based on these results, we proposed a novel iCa prediction equation.</p> Results <p>Two hundred and thirteen patients were enrolled. The median patient age was 65 (standard deviation, 10.2) years, and <Emphasis Type="Underline">68.2</Emphasis>% of patients were male. The correlation coefficients were 0.867 for iCa and tCa and 0.854 for iCa and cCa. Correlations were also observed when albumin concentrations and pH were divided, both of which are known correlation factors. In multiple regression analysis of the relationship between tCa and iCa, sex, pH, albumin, phosphate, and magnesium levels were significant factors. We established a new corrected calcium equation using age and albumin level: (mg/dL) = 0.828 × total calcium − 0.314 × albumin (g/dL) + 0.007 × age + 2.922.</p> Conclusions <p>iCa, tCa, and cCa had a correlation in patients undergoing hemodialysis, with a stronger correlation with tCa in iCa assessments.</p>