For educational purposes only. Consult your doctor for diagnosis and medical advice.
A1C is calculated using the ADA ADAG formula, derived from the 2008 ADAG study in Diabetes Care. The study correlated lab-measured A1C with continuous glucose monitor averages across 507 adults over 3 months:
Worked example: Average glucose of 126 mg/dL over 3 months.
The formula requires a 3-month average, not a single reading. A single fasting glucose measurement will produce a misleading estimate. For other clinical lab value conversions, the Corrected Calcium Calculator uses a similar approach for lab value adjustment.
Glucose Management Indicator (GMI) is a CGM-derived value that estimates what an A1C might be from sensor data. It uses a separate formula published in Diabetes Care (2018):
GMI and lab A1C are related but not interchangeable. The table below shows key differences:
| Factor | GMI | Lab A1C |
|---|---|---|
| Source | CGM sensor data | Lab blood test |
| Time window | Typically 14-90 days of CGM data | Approximately 3 months (red cell lifespan) |
| Affected by hemoglobin variants? | No | Yes (can be falsely high or low) |
| Typical divergence | 0.3 to 0.5 percentage points from lab A1C | Reference standard |
| Used for diagnosis? | No, not a diagnostic test | Yes, 6.5% on two tests |
If your GMI and lab A1C consistently differ by more than 0.5 percentage points, discuss it with your endocrinologist. Conditions that affect red blood cell turnover (hemolytic anemia, iron deficiency, kidney disease) can widen this gap significantly. For biological age markers tied to glucose metabolism, see how the Chronological Age Calculator is used alongside HbA1c in biological age research.
The full A1C to eAG conversion table below covers the clinically relevant range. Values are calculated using the ADA formula and match the conversion published in Diabetes Care:
| A1C % | eAG mg/dL | eAG mmol/L | Range |
|---|---|---|---|
| 5.0% | 97 | 5.4 | Normal |
| 5.5% | 111 | 6.2 | Normal |
| 5.7% | 117 | 6.5 | Prediabetes |
| 6.0% | 126 | 7.0 | Prediabetes |
| 6.5% | 140 | 7.8 | Diabetes |
| 7.0% | 154 | 8.5 | Diabetes |
| 7.5% | 169 | 9.4 | Diabetes |
| 8.0% | 183 | 10.2 | Diabetes |
| 9.0% | 212 | 11.8 | High risk |
| 10.0% | 240 | 13.4 | High risk |
For patients with conditions that make A1C unreliable, fructosamine is the alternative short-term marker. The conversion from fructosamine (micromol/L) to estimated A1C is: A1C (%) = (fructosamine / 59.7) + 1.97. This is an approximation and varies by laboratory method. For other clinical calculations used in lab interpretation, the Anion Gap Calculator covers another commonly ordered metabolic panel value.
Example 1 (Glucose to A1C): A CGM report shows an average of 154 mg/dL over 90 days.
Example 2 (A1C to eAG): Lab report shows an A1C of 5.5%.