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How to Calculate A1C — ADAG Formula Explained
How to convert A1C to average blood glucose and calculate A1C from glucose readings.
If you've ever wondered how doctors turn an A1C percentage into an average blood sugar number, the answer is the ADAG formula. It's a simple equation backed by a landmark study, and it's the standard used worldwide for this conversion.
The Formula
eAG (mg/dL) = 28.7 × A1C − 46.7
eAG (mmol/L) = 1.59 × A1C − 2.59
eAG stands for estimated average glucose — the average blood sugar level over the past 2 to 3 months, which is the lifespan of a red blood cell.
What Is the ADAG Study?
ADAG stands for A1C-Derived Average Glucose. The study was published in 2008 by David M. Nathan and colleagues in Diabetes Care. It was designed to answer a simple question: what average blood sugar does a given A1C actually represent?
Previous conversion formulas existed, but they were based on limited data. The ADAG study enrolled 507 participants — including people with type 1 diabetes, type 2 diabetes, and non-diabetics — across 10 international centers. Each participant wore a continuous glucose monitor (CGM) for 48 hours per month and performed daily finger-stick tests over 3 months.
The result was a tight linear relationship between A1C and average glucose, with a correlation coefficient (R²) of 0.84 — meaning A1C explained 84% of the variance in average glucose.
Quick Examples
| A1C | Calculation | eAG (mg/dL) | eAG (mmol/L) |
|---|---|---|---|
| 5.0% | 28.7 × 5.0 − 46.7 | 97 | 5.4 |
| 6.0% | 28.7 × 6.0 − 46.7 | 126 | 7.0 |
| 7.0% | 28.7 × 7.0 − 46.7 | 154 | 8.6 |
| 8.0% | 28.7 × 8.0 − 46.7 | 183 | 10.2 |
| 10.0% | 28.7 × 10.0 − 46.7 | 240 | 13.3 |
For a full conversion table, see the HbA1c chart.
Why Use eAG Instead of A1C?
A1C is reported as a percentage, which can feel abstract. Most people with diabetes check their blood sugar in mg/dL or mmol/L daily. Expressing A1C as an average glucose in the same units makes it easier to connect lab results with everyday readings.
For example, an A1C of 7.0% might not mean much to a patient. But saying "your average blood sugar over the last 3 months has been about 154 mg/dL" is immediately relatable.
Limitations
The ADAG formula is a population average — individual results can vary. Several factors can make A1C less reliable:
- Red blood cell turnover: Conditions like anemia, blood loss, or transfusions change RBC lifespan and skew A1C.
- Hemoglobin variants: Sickle cell trait or other hemoglobinopathies can interfere with some A1C assays.
- Pregnancy: Increased red blood cell production can lower A1C values.
- Ethnicity: The ADAG study found small but measurable differences across ethnic groups at the same average glucose.
- Glycemic variability: Two people with the same average glucose but different amounts of highs and lows will have the same eAG but potentially different health outcomes.
ADAG vs. Older Formulas
Before ADAG, the most common conversion came from the DCCT (Diabetes Control and Complications Trial), which used the formula: eAG = 35.6 × A1C − 77.3. This was based on a smaller, less diverse group (mostly type 1 diabetes patients using finger-stick data only).
The ADAG formula is now the standard recommended by the ADA (American Diabetes Association) and used by most labs and calculators worldwide — including this one.
References
- Nathan DM, Kuenen J, Borg R, Zheng H, Schoenfeld D, Heine RJ. Translating the A1C Assay Into Estimated Average Glucose Values. Diabetes Care. 2008;31(8):1473-1478.
- American Diabetes Association Professional Practice Committee. Glycemic Goals and Hypoglycemia: Standards of Care in Diabetes—2024. Diabetes Care. 2024;47(Suppl 1):S111-S125.
- Diabetes Control and Complications Trial Research Group. The Effect of Intensive Treatment of Diabetes on the Development and Progression of Long-Term Complications in Insulin-Dependent Diabetes Mellitus. N Engl J Med. 1993;329:977-993.