Our method
Every calculator on this site uses a published formula. Here they are, with the rounding rules and the tests that check them.
Formulas
- A1c to estimated average glucose (eAG): eAG mg/dL = 28.7 × A1c − 46.7 (Nathan 2008), the formula used in the ADA calculator (ADA eAG).
- mg/dL to mmol/L: divide by 18.016.
- A1c % and IFCC mmol/mol: % = 0.09148 × mmol/mol + 2.152, the NGSP master equation (NGSP).
- GMI: GMI % = 3.31 + 0.02392 × mean glucose mg/dL (Bergenstal 2018). With mmol/L input we convert to mg/dL first, so both units give the same answer.
- Categories: normal below 5.7%, prediabetes 5.7% to 6.4%, diabetes 6.5% or higher (ADA Standards 2026, sec. 2). The A1c is rounded to one decimal before it is placed in a category, as labs report it.
Rounding
mg/dL to whole numbers, mmol/L and A1c to one decimal, IFCC to whole numbers. Displayed values match the ADA eAG table (for example 7% = 154 mg/dL, 6% = 126 mg/dL).
Testing
The calculation code is one small module with automated tests: eAG against the ADA table for 5% to 12%, IFCC against standard reference points (6.5% = 48 mmol/mol, 7% = 53 mmol/mol), GMI against worked values, and the category cut points at 5.6, 5.7, 6.4 and 6.5.
Privacy by design
Calculations run in your browser. Nothing you type is sent anywhere. See privacy.
Sources
- Nathan DM, Kuenen J, Borg R, Zheng H, Schoenfeld D, Heine RJ; A1c-Derived Average Glucose (ADAG) Study Group. Translating the A1C assay into estimated average glucose values. Diabetes Care. 2008;31(8):1473-1478. doi.org/10.2337/dc08-0545
- American Diabetes Association. eAG/A1C conversion calculator (professional.diabetes.org). professional.diabetes.org/glucose_calc
- NGSP. IFCC Standardization of HbA1c: the IFCC and NGSP master equation (NGSP = 0.09148 x IFCC + 2.152). ngsp.org/ifccngsp.asp
- Bergenstal RM, Beck RW, Close KL, et al. Glucose Management Indicator (GMI): a new term for estimating A1C from continuous glucose monitoring. Diabetes Care. 2018;41(11):2275-2280. doi.org/10.2337/dc18-1581
- American Diabetes Association Professional Practice Committee. 2. Diagnosis and Classification of Diabetes: Standards of Care in Diabetes 2026. Diabetes Care. 2026;49(Suppl 1). doi.org/10.2337/dc26-S002
Written and maintained by the controla1clevels.com editorial team. We do not have a medical reviewer; every claim links to its primary source above. Last reviewed: 2026-10-25. Spotted an error? See About.