The ADAG formula
All conversions use the estimated average glucose formula from the ADAG study, adopted by the American Diabetes Association:
| Conversion | Formula | Example |
|---|---|---|
| A1C to eAG (mg/dL) | eAG = 28.7 × A1C − 46.7 | A1C 7.0% → 154 mg/dL |
| eAG to A1C | A1C = (eAG + 46.7) ÷ 28.7 | 140 mg/dL → 6.5% |
| mg/dL to mmol/L | mmol/L = mg/dL ÷ 18.0182 | 154 mg/dL → 8.6 mmol/L |
| mmol/L to mg/dL | mg/dL = mmol/L × 18.0182 | 7.8 mmol/L → 140 mg/dL |
Because the relationship is linear, each full 1% of A1C corresponds to exactly 28.7 mg/dL of average glucose. That property is what makes the full conversion chart trustworthy across its entire range.
Reference range classification
The result panel classifies A1C into three clinical ranges using the standard diagnostic thresholds: below 5.7% normal, 5.7% to 6.4% prediabetes, and 6.5% or higher diabetes. In eAG terms those boundaries are approximately 117 mg/dL and 140 mg/dL. Classification is for education and matches how a typical clinic would read the result, but diagnosis is always a clinician's decision.
Average glucose to A1C estimation
The average-glucose mode sums the readings you enter, divides by the count, and feeds the average through the reverse ADAG formula. A handful of readings over a day or two is not a reliable A1C estimate; the more readings spread over weeks, the closer the estimate gets to a lab A1C. The tool is designed to be used that way, and the note on the calculator says so.
Known limitations
The ADAG formula is a population average. Individual variation is real: red blood cell lifespan, anemia, recent blood loss, kidney disease, some hemoglobin variants, and pregnancy can all make a person's true glucose-to-A1C relationship differ from the formula. The conversion is a communication tool and a tracking aid, not a substitute for a lab A1C or for clinical judgment.
Methodology questions
What formula does the A1C calculator use?
The ADAG formula: eAG (mg/dL) = 28.7 × A1C − 46.7. To convert eAG back to A1C: A1C = (eAG + 46.7) ÷ 28.7. For mmol/L, divide the mg/dL value by 18.0182.
What is the estimated average glucose (eAG)?
eAG is the average blood glucose an A1C percentage corresponds to, expressed in the same mg/dL units as a blood glucose meter. A 7.0% A1C equals an eAG of about 154 mg/dL.
Where do the A1C reference ranges come from?
The standard clinical classification: below 5.7% normal, 5.7% to 6.4% prediabetes, and 6.5% or higher diabetes, as used by the American Diabetes Association.
How is A1C estimated from average blood glucose?
The calculator averages the glucose readings you enter, then applies A1C = (eAG + 46.7) ÷ 28.7. A single reading is not a valid basis for an A1C estimate; several readings over weeks are needed.