A1C 6.2% = an estimated average glucose of 131 mg/dL, or 7.3 mmol/L.
It also sits in the prediabetes band. Those are two different facts and it is worth keeping them apart — one is a unit conversion, the other is a diagnostic category.
The conversion table
| A1C | eAG (mg/dL) | eAG (mmol/L) |
|---|---|---|
| 5.0% | 97 | 5.4 |
| 5.5% | 111 | 6.2 |
| 5.7% | 117 | 6.5 |
| 6.0% | 126 | 7.0 |
| 6.2% | 131 | 7.3 |
| 6.5% | 140 | 7.8 |
| 7.0% | 154 | 8.6 |
| 7.5% | 169 | 9.4 |
| 8.0% | 183 | 10.1 |
| 9.0% | 212 | 11.8 |
| 10.0% | 240 | 13.4 |
| 12.0% | 298 | 16.5 |
The formula, from the ADAG study, is straightforward:
eAG (mg/dL) = 28.7 × A1C − 46.7
eAG (mmol/L) = eAG (mg/dL) ÷ 18
What the bands mean
| A1C | Category |
|---|---|
| Below 5.7% | Normal |
| 5.7% – 6.4% | Prediabetes |
| 6.5% and above | Diabetes |
6.2% is in the upper half of the prediabetes band. The thresholds are lines drawn across a continuous scale, so 6.4% and 6.5% are not meaningfully different measurements — they are on either side of an administrative boundary. A diagnosis normally requires two abnormal results, or one plus a confirming test.
What A1C actually measures
Glucose attaches to haemoglobin in red blood cells, irreversibly, at a rate proportional to how much glucose is present. Red cells live about three months, so the proportion of glycated haemoglobin reflects average exposure over roughly that window.
The window is weighted toward the recent past: the last 30 days contribute roughly half the result, the 60 to 120-day range considerably less. A good month after a poor quarter shows up partially; a poor fortnight before the test also shows.
Four reasons A1C can be wrong
Because the test measures red blood cells rather than glucose directly, anything that changes red cell lifespan changes the result — without anything changing about the blood sugar.
Shortened red cell lifespan reads LOW. Haemolytic anaemia, recent significant blood loss, recent transfusion, chronic kidney disease with EPO treatment, pregnancy in the second and third trimesters.
Lengthened red cell lifespan reads HIGH. Iron deficiency anaemia, B12 deficiency, splenectomy.
Haemoglobin variants interfere with the assay. HbS, HbC, HbE traits — common in people of African, Mediterranean, Middle Eastern and Southeast Asian descent. Some assays handle them, some do not; the lab needs to know.
Averages hide variability. An A1C of 6.2% is compatible with steady readings around 130, and also with swings between 70 and 200. The average is identical, the clinical picture is not. That gap is why continuous monitoring reports time in range alongside average.
What eAG is for
The conversion exists so a percentage can be compared with the numbers on a home meter, which read in mg/dL or mmol/L. It is the same information on a familiar scale.
It is an estimate, not a prediction of any individual reading. A morning fingerstick of 105 with an A1C of 6.2% is not a contradiction — fasting glucose is normally the lowest reading of the day.
Other lab values move between units on their own conversion factors, and none of them share glucose's divisor. Vitals are read against fixed category thresholds rather than reference intervals.
Sources: American Diabetes Association, Standards of Care in Diabetes, diagnostic criteria and eAG conversion; Nathan et al., ADAG study, Diabetes Care (translating A1C to average glucose); NIDDK, The A1C Test and Diabetes.
Reference information only, not medical advice. An A1C result belongs with the clinician who ordered it, together with the rest of your history.