The study aimed to establish a mathematical relationship between the A1C assay, which measures chronic glycemia, and average glucose (AG) levels, to determine if A1C could be expressed in the same units as self-monitoring of capillary glucose concentrations. A total of 507 subjects, including patients with type 1 and type 2 diabetes and nondiabetic individuals, were included. A1C levels were compared with AG levels calculated from continuous glucose monitoring and self-monitoring of capillary glucose. Linear regression analysis revealed a strong correlation (AGavg = 28.7 × A1C − 46.7, R² = 0.84, P < 0.0001), allowing for the calculation of estimated average glucose (eAG) from A1C values. The relationship between A1C and AG was consistent across different subgroups based on age, sex, diabetes type, race/ethnicity, and smoking status. The study concluded that A1C levels can be effectively translated into eAG for most patients with type 1 and type 2 diabetes, providing a more clinically useful index of chronic glycemia.The study aimed to establish a mathematical relationship between the A1C assay, which measures chronic glycemia, and average glucose (AG) levels, to determine if A1C could be expressed in the same units as self-monitoring of capillary glucose concentrations. A total of 507 subjects, including patients with type 1 and type 2 diabetes and nondiabetic individuals, were included. A1C levels were compared with AG levels calculated from continuous glucose monitoring and self-monitoring of capillary glucose. Linear regression analysis revealed a strong correlation (AGavg = 28.7 × A1C − 46.7, R² = 0.84, P < 0.0001), allowing for the calculation of estimated average glucose (eAG) from A1C values. The relationship between A1C and AG was consistent across different subgroups based on age, sex, diabetes type, race/ethnicity, and smoking status. The study concluded that A1C levels can be effectively translated into eAG for most patients with type 1 and type 2 diabetes, providing a more clinically useful index of chronic glycemia.