What A1C Actually Measures
Your HbA1c is the percentage of hemoglobin molecules with glucose stuck to them. Because red blood cells live about three months, A1C reflects your average glucose over roughly the past 8–12 weeks — a smoothed picture, not a snapshot. The estimated average glucose (eAG) translates that percentage back into the mg/dL or mmol/L numbers you see on a meter.
A1C to Blood Sugar Chart
| HbA1c (%) | eAG (mg/dL) | eAG (mmol/L) | Category |
|---|---|---|---|
| 5.0 | 97 | 5.4 | Optimal |
| 5.4 | 108 | 6.0 | Normal |
| 5.6 | 114 | 6.3 | Top of normal |
| 5.7 | 117 | 6.5 | Prediabetes |
| 6.0 | 126 | 7.0 | Prediabetes |
| 6.4 | 137 | 7.6 | Prediabetes |
| 6.5 | 140 | 7.8 | Diabetes |
| 7.0 | 154 | 8.6 | Diabetes |
| 8.0 | 183 | 10.2 | Above target |
| 9.0 | 212 | 11.8 | Above target |
| 10.0 | 240 | 13.4 | Above target |
The Formula
The conversion comes from the ADAG study and is simple enough to do by hand:
eAG (mg/dL) = 28.7 × A1C − 46.7
So an A1C of 6.2% is 28.7 × 6.2 − 46.7 ≈ 131 mg/dL. For mmol/L, divide by 18 (131 ÷ 18 ≈ 7.3). A useful rule of thumb: each whole 1% of A1C is worth about 29 mg/dL of average glucose.
Why Your Meter and A1C Can Disagree
eAG is an average. Two people with an identical A1C of 6.5% can have very different days — one steady around 140 mg/dL, the other swinging from 90 to 220. That's why A1C and a single finger-stick rarely match exactly.
A1C can also mislead when red-blood-cell lifespan is abnormal:
- Reads falsely low: recent blood loss, hemolytic anemia, pregnancy, high-dose vitamin C or E.
- Reads falsely high: iron-deficiency anemia, kidney disease, certain hemoglobin variants.
If your A1C and meter consistently tell different stories, that gap is itself worth investigating.
A1C Is the Average — Watch the Spikes Too
Because A1C hides volatility, pair it with a fasting glucose and, ideally, a sense of your post-meal numbers. To see how hard your body works to keep that average down, add fasting insulin and HOMA-IR — high insulin with a normal A1C is the earliest metabolic warning.
Keep the Whole Picture on One Timeline
A1C, eAG, fasting glucose, and insulin are most useful together and over time. Merios converts and tracks them automatically, so you read your metabolic trend at a glance — and estimate your biological age from the same panel.
Related: HbA1c 5.7% · HbA1c 6.0% · HbA1c 6.5% · How to lower A1C
This article is educational and not medical advice. Interpret your results with a qualified clinician.