To convert average glucose to A1C, use the American Diabetes Association’s ADAG formula: A1C (%) = (estimated average glucose + 46.7) / 28.7. Plug in your meter or CGM mean in mg/dL and you get the expected A1C. The formula works in reverse too, letting you translate a lab A1C into an average blood sugar number.
The ADAG Formula in Plain English
In 2008, a multicenter study led by David Nathan at Massachusetts General Hospital correlated 2,700 glucose readings per participant (from continuous monitors plus 8-point fingerstick days) against A1C in 507 adults across four ethnic groups. The linear regression that emerged is now the calculation every US lab uses when it prints “estimated average glucose” on your A1C result.
The math looks like this:
- Glucose to A1C: A1C (%) = (eAG + 46.7) / 28.7
- A1C to glucose: eAG (mg/dL) = 28.7 x A1C – 46.7
- Glucose to A1C (mmol/L): A1C (%) = (eAG + 2.59) / 1.59
Example: if your 30-day meter average is 140 mg/dL, your expected A1C is (140 + 46.7) / 28.7 = 6.5 percent. See our A1C levels hub for where that falls in the diagnostic range.
Average Glucose to A1C Conversion Chart
| Average Glucose (mg/dL) | Average Glucose (mmol/L) | A1C (%) | Category |
|---|---|---|---|
| 97 | 5.4 | 5.0 | Normal |
| 108 | 6.0 | 5.4 | Normal |
| 114 | 6.3 | 5.6 | Normal (upper) |
| 117 | 6.5 | 5.7 | Prediabetes (lower) |
| 126 | 7.0 | 6.0 | Prediabetes |
| 137 | 7.6 | 6.4 | Prediabetes (upper) |
| 140 | 7.8 | 6.5 | Diabetes (confirmed x2) |
| 154 | 8.6 | 7.0 | Diabetes |
| 183 | 10.2 | 8.0 | Diabetes (uncontrolled) |
| 212 | 11.8 | 9.0 | Diabetes (uncontrolled) |
Why the Conversion Is Not Exact
A1C is not a direct measurement of glucose. It measures what percentage of hemoglobin molecules in your red blood cells have been non-enzymatically glycated by circulating sugar. Because red cells live roughly 120 days, A1C reflects a 90-day weighted average, with the most recent 30 days contributing about half the signal.
The ADAG formula assumes a typical red-cell lifespan. People with iron deficiency anemia, hemolytic anemia, recent blood loss, chronic kidney disease, or certain hemoglobin variants can have A1C values that drift from their true average glucose by 0.5 percent or more. That is why a CGM-based Glucose Management Indicator often outperforms A1C for day-to-day management, particularly in people with anemia or hemoglobinopathies.
Meter Averages vs Lab A1C
If your home glucose meter 30-day average says 120 mg/dL but your lab A1C comes back 6.4 percent (eAG equivalent 137 mg/dL), the discrepancy is almost always sampling bias, not lab error. Most people test when they remember to test: fasting in the morning, occasionally 2 hours after a meal, rarely mid-afternoon or overnight. Overnight glucose in people with insulin resistance tends to rise slowly between 2 and 6 a.m. (the “dawn phenomenon“), which your meter never captures but your hemoglobin records faithfully.
A 14-day continuous glucose monitor trial, which many prediabetes and diabetes programs now include, samples glucose every 5 minutes. According to Bergenstal et al. in Diabetes Care (2018), the Glucose Management Indicator (GMI) calculated from CGM data correlates with lab A1C within 0.3 percent for 65 percent of people and within 0.5 percent for 95 percent of people. When GMI and A1C disagree by more than 0.5 percent, the pattern is usually reproducible for that individual.
How to Use the Conversion Practically
Between A1C tests. Most doctors order A1C every 3-6 months for prediabetes and 3 months for diabetes. In the gap, your meter or CGM mean gives you a running forecast. If you are working to lower A1C from 6.2 to 5.6, your goal eAG is roughly 114 mg/dL. A 2-week rolling mean close to 114 predicts you will hit it.
Troubleshooting spikes. If your fasting glucose runs 100 but your A1C comes back 6.0 (eAG 126), post-meal spikes are driving the gap. Measure 2 hours after your largest carb meals for a week to find the culprit. See our diet and nutrition guide for carb-swap strategies that reduce post-meal peaks.
Validating a home meter. Meters carry 10-15 percent measurement error. If your meter average consistently predicts an A1C 0.5 percent lower than lab readings across two or three cycles, consider calibration or replacement.
When the Formula Breaks Down
The ADAG equation was derived in adults without significant anemia, kidney disease, or hemoglobin variants. According to the ADA Standards of Care 2024, conditions that can make A1C unreliable include:
- Iron deficiency anemia (often falsely raises A1C)
- Hemolytic anemia and sickle cell trait (often falsely lowers A1C)
- Recent transfusion (falsely lowers A1C)
- Chronic kidney disease stage 4-5 (variable effect)
- Pregnancy (A1C underestimates glucose due to faster red-cell turnover)
In these cases, fructosamine, glycated albumin, or direct CGM monitoring may be preferred over the ADAG formula.
The Bottom Line
Average glucose converts to A1C using the formula A1C (%) = (eAG + 46.7) / 28.7. The chart above covers the clinically relevant range from normal (under 5.7 percent) through prediabetes (5.7-6.4) and into diabetes (6.5 and above). Use the conversion to interpret your meter or CGM data between lab visits, to troubleshoot discrepancies, and to track progress. For most people without anemia or hemoglobin disorders, the formula is accurate within 0.3 percent.
