HbA1c (often written A1C) estimates your average blood glucose over roughly the past 2–3 months. It is not calculated from a single fasting or post-meal glucose reading; a lab measures the percentage of hemoglobin in red blood cells that has glucose attached to it.
A common conversion is:
Estimated Average Glucose (eAG, mg/dL) = 28.7 × A1C − 46.7
So the reverse is:
A1C (%) = (eAG + 46.7) ÷ 28.7
For example, if your average glucose is about 154 mg/dL, then:
A1C ≈ (154 + 46.7) ÷ 28.7 ≈ 7.0%
Approximate equivalents:
| HbA1c | Average glucose |
|---|---|
| 5.7% | 117 mg/dL / 6.5 mmol/L |
| 6.0% | 126 mg/dL / 7.0 mmol/L |
| 6.5% | 140 mg/dL / 7.8 mmol/L |
| 7.0% | 154 mg/dL / 8.6 mmol/L |
| 8.0% | 183 mg/dL / 10.2 mmol/L |
| 9.0% | 212 mg/dL / 11.8 mmol/L |
| 10.0% | 240 mg/dL / 13.4 mmol/L |
If you give me your recent fasting/post-meal glucose readings, I can estimate what HbA1c they roughly correspond to.