Estimated average glucose (eAG) is a milligram-per-deciliter (mg/dL) translation of your A1C percentage. To convert eAG back to A1C, use the ADAG formula: A1C (%) = (eAG + 46.7) / 28.7. For example, an eAG of 154 mg/dL converts to an A1C of about 7.0%. The conversion makes CGM and meter data directly comparable to lab A1C results.
Where the Formula Comes From
The A1C-Derived Average Glucose (ADAG) study, published by Nathan and colleagues in Diabetes Care (2008), correlated continuous glucose monitoring and frequent fingerstick readings with lab A1C in 507 adults across the diabetes spectrum. The resulting regression is the ADA-endorsed conversion:
- A1C → eAG: eAG (mg/dL) = 28.7 × A1C − 46.7
- eAG → A1C: A1C (%) = (eAG + 46.7) / 28.7
eAG to A1C Conversion Chart
| eAG (mg/dL) | A1C (%) | Category |
|---|---|---|
| 97 | 5.0% | Normal |
| 111 | 5.5% | Normal (upper end) |
| 117 | 5.7% | Prediabetes (threshold) |
| 126 | 6.0% | Prediabetes |
| 140 | 6.5% | Diabetes (threshold) |
| 154 | 7.0% | Diabetes |
| 183 | 8.0% | Diabetes (above target) |
| 212 | 9.0% | Diabetes (poorly controlled) |
| 240 | 10.0% | Diabetes (poorly controlled) |
See our A1C levels guide for interpretive detail across the range.
eAG vs GMI vs Meter Average
Three different “average glucose” numbers circulate in diabetes care:
- eAG: Derived from lab A1C. Best thought of as “A1C in mg/dL units.”
- GMI (Glucose Management Indicator): Derived directly from CGM data using a separate formula (Bergenstal 2018). It estimates what your lab A1C likely is — but biology varies, so GMI and lab A1C can differ by 0.3-0.5%.
- Meter average: The arithmetic mean of your fingerstick values. Biased by when you test.
How to Use eAG in Real Life
CGM Users
Modern CGMs (Dexcom G7, Libre 3, Guardian Connect) display GMI and sometimes an eAG estimate. If your GMI is 6.8% but your lab A1C is 7.1%, the 0.3% gap is within expected biological variability.
Fingerstick Meter Users
If your meter reports a 14-day average of 145 mg/dL, the eAG-to-A1C conversion gives 6.7%. But fingerstick averages are less representative because they skip overnight and post-meal spikes. Use the number as a rough guide only.
Limitations of the Conversion
The ADAG formula is an average across a population, not a perfect individual predictor. Individual differences matter:
- Red blood cell lifespan: shorter lifespan (anemia, hemolysis) lowers A1C relative to eAG; longer lifespan raises it
- Hemoglobin variants: sickle cell trait, thalassemia, and others can interfere with A1C assays
- Race and ethnicity: small but measurable differences in glycation rates have been documented
- CGM calibration: lag between interstitial and blood glucose, plus sensor drift
When to Trust Which Number
| Situation | Best metric |
|---|---|
| Formal diagnosis of prediabetes or diabetes | NGSP-certified lab A1C |
| Day-to-day glucose management | CGM time-in-range and GMI |
| Comparing CGM data to prior A1C | eAG or GMI from CGM |
| Between lab visits | Home A1C meter or CGM |
How the ADAG Study Was Designed
Understanding the study behind the formula clarifies where the conversion works and where it breaks. Nathan and colleagues recruited 507 adults across 10 international centers: 268 with type 1 diabetes, 159 with type 2 diabetes, and 80 non-diabetic controls. Each participant wore a continuous glucose monitor for at least two 2-day periods and took 7-point fingerstick profiles on 3 days per week over 12 weeks, yielding roughly 2,700 glucose values per person. Simultaneous A1C was measured centrally using an NGSP-certified high-performance liquid chromatography assay.
The resulting linear regression — eAG = 28.7 x A1C – 46.7 — had an R-squared of 0.84, meaning A1C explains about 84% of the variability in average glucose across the population. The remaining 16% reflects individual biology, which is why a formula-based estimate cannot replace direct measurement.
Ethnic Variation and the Hemoglobin Glycation Index
The ADAG study did not find a statistically significant ethnic difference in the A1C-to-eAG relationship, but later and larger analyses have. The Diabetes Prevention Program and NHANES both show that Black, Hispanic, and Asian American adults run A1C approximately 0.2-0.4% higher than non-Hispanic white adults at equivalent fasting and post-challenge glucose levels.
Researchers describe this through the hemoglobin glycation index (HGI) — the difference between measured A1C and what would be predicted from glucose alone. A positive HGI means your red blood cells glycate faster than average; a negative HGI means slower. HGI is reproducible within individuals and partially heritable. Clinically, it means two people with identical CGM averages can have lab A1Cs that differ by 0.5% or more, without either being wrong.
When eAG Misleads
Situations where the ADAG formula gives a misleading answer:
- Shortened red blood cell lifespan: hemolysis, recent blood loss, chronic kidney disease on erythropoietin, late pregnancy — A1C underestimates true average glucose, so back-calculating an eAG gives a falsely low number.
- Lengthened lifespan: iron deficiency anemia, splenectomy, aplastic anemia — A1C overestimates average glucose.
- Hemoglobin variants: HbS, HbC, HbE, HbF; interference depends on the specific assay. NGSP lists affected methods at ngsp.org.
- High glucose variability: Very frequent hypoglycemia with post-meal spikes can produce an “average” that hides dangerous swings; A1C looks reassuring but clinical risk is high.
- Recent rapid glucose change: eAG assumes steady state. Someone who dropped their average from 180 to 120 mg/dL last month will have a lagging A1C that overstates current control.
What to Do with the Conversion
If your eAG or GMI suggests an A1C in the prediabetes range, confirm with a lab test. Lifestyle interventions from the Diabetes Prevention Program — losing 5%-7% of body weight and 150 minutes of weekly activity — reduced progression to type 2 diabetes by 58% over 3 years. Visit our treatment hub for structured options.
The Bottom Line
eAG to A1C conversion is a simple arithmetic exercise — but the clinical interpretation requires context. Use the ADAG formula to translate your CGM or meter averages into familiar A1C units, then verify meaningful changes with a lab test. The conversion is a helpful bridge between daily monitoring and the 3-month snapshot your doctor uses for diagnosis and treatment decisions.
