A1C Calculator
Convert between HbA1c and estimated average glucose (eAG) using the ADAG study equation. Calculate your A1C or find out what your A1C means in everyday glucose numbers.
A1c → Average Glucose
Average Glucose → A1c
Disclaimer: For educational purposes only. Not a substitute for clinical judgment.
About This Tool
What Is HbA1c and How Is A1C Calculated?
Hemoglobin A1c (HbA1c or simply A1c) measures the percentage of hemoglobin that is glycated — bound to glucose. Because red blood cells have a lifespan of approximately 120 days, A1c reflects average blood glucose over the preceding 2–3 months, with greater weighting toward the most recent 30 days. A1c is the standard biomarker for monitoring glycemic control in diabetes and is used for both diagnosis (≥ 6.5% = diabetes) and treatment target assessment.
The ADAG Study Equation
The A1c-Derived Average Glucose (ADAG) study, published by Nathan et al. in Diabetes Care (2008), established the linear relationship between A1c and estimated average glucose (eAG): eAG (mg/dL) = 28.7 × A1c − 46.7. This equation was derived from 507 participants (including type 1, type 2, and non-diabetic individuals) who wore continuous glucose monitors and had frequent self-monitoring over 3 months. The correlation was strong (r = 0.92), enabling the ADA and AACE to recommend reporting eAG alongside A1c to improve patient understanding.
A1C Interpretation: What Your A1C Number Means
While A1c is the gold standard for monitoring glycemic control, several conditions can cause discordance between A1c and true average glucose. Conditions that shorten red blood cell lifespan (hemolytic anemias, chronic kidney disease, recent blood transfusion) tend to falsely lower A1c. Conditions that prolong red blood cell lifespan (iron deficiency anemia, vitamin B12 deficiency, splenectomy) tend to falsely elevate A1c. Hemoglobin variants (HbS, HbC, HbE) can interfere with certain A1c assays. In these situations, fructosamine or CGM-derived GMI may be more reliable alternatives.
🔑 Clinical Pearls
- A1c < 7.0% is the general ADA target for most non-pregnant adults; individualize based on hypoglycemia risk, life expectancy, and comorbidities.
- A1c does not capture glycemic variability — two patients with the same A1c can have very different glucose excursion patterns.
- A 1% change in A1c corresponds to approximately a 29 mg/dL (1.6 mmol/L) change in eAG.
- CGM metrics (Time in Range, GMI) are increasingly used alongside or instead of A1c for diabetes management.
Key References
- Nathan DM, et al. Translating the A1C assay into estimated average glucose values. Diabetes Care. 2008;31(8):1473-1478.
- American Diabetes Association. Standards of Care in Diabetes — 2024. Diabetes Care. 2024;47(Suppl 1).
- Bergenstal RM, et al. Glucose management indicator (GMI): a new term for estimating A1C from continuous glucose monitoring. Diabetes Care. 2018;41(11):2275-2280.
Formula last verified: February 2026