Enter your CGM average glucose or A1C to estimate the other value. The ADAG regression is used, which closely approximates the Glucose Management Indicator (GMI) near typical glucose levels. Perform a quick conversion with one of the links below.
Note: This calculator provides an estimate based on population data (ADAG). It is not a diagnosis and does not replace an A1C blood test. A1C and average glucose can disagree in some situations (anemia, hemoglobin variants, pregnancy, kidney disease, or recent blood loss/transfusion); discuss results with a clinician.
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GMI and ADAG: Two Formulas for Estimating A1C
Two distinct regression formulas convert mean glucose to an estimated A1C. The GMI formula (Bergenstal et al., Diabetes Care 41:11, 2018) was derived from CGM sensor data across a population with Type 1 and Type 2 diabetes. The ADAG formula (Nathan et al., Diabetes Care 31:8, 2008) was derived from self-monitored finger-stick readings paired with laboratory A1C. This calculator uses ADAG for bidirectional conversion; the GMI formula is shown for comparison.
\text{GMI (\%)} = 3.31 + 0.02392 \times \text{Mean Glucose}_{mg/dL}\text{ADAG A1C (\%)} = (\text{Mean Glucose}_{mg/dL} + 46.7)\, /\, 28.7The two formulas yield identical results at exactly 154 mg/dL (8.6 mmol/L). Below that crossover, GMI returns a higher percentage than ADAG; above it, ADAG returns a higher percentage. The divergence can reach more than 1% at glucose extremes.
| Mean Glucose (mg/dL) | GMI % (Bergenstal 2018) | ADAG A1C % (Nathan 2008) | Difference (GMI minus ADAG) |
|---|---|---|---|
| 70 | 4.98 | 4.07 | +0.91 (GMI higher) |
| 90 | 5.46 | 4.76 | +0.70 (GMI higher) |
| 110 | 5.94 | 5.46 | +0.48 (GMI higher) |
| 130 | 6.42 | 6.16 | +0.26 (GMI higher) |
| 154 (crossover point) | 6.99 | 6.99 | 0.00 |
| 170 | 7.38 | 7.55 | -0.17 (ADAG higher) |
| 200 | 8.09 | 8.59 | -0.50 (ADAG higher) |
| 230 | 8.81 | 9.64 | -0.83 (ADAG higher) |
| 260 | 9.53 | 10.69 | -1.16 (ADAG higher) |
| GMI = 3.31 + 0.02392 x mean glucose (mg/dL). ADAG = (mean glucose + 46.7) / 28.7. The two formulas converge at 154 mg/dL (8.6 mmol/L). Below that crossover, GMI yields a higher number; above it, ADAG yields a higher number. | |||
6.3 GMI to A1C
6.3 GMI is about 5.98% A1C. On this page’s formula set, that corresponds to an estimated mean glucose of about 125 mg/dL.
In IFCC units, this is about 42 mmol/mol. Formula path: 6.3 GMI → 125.00 mg/dL → 5.98% A1C.
5.7 GMI to A1C
5.7 GMI is about 5.11% A1C. On this page’s formula set, that corresponds to an estimated mean glucose of about 99.92 mg/dL.
In IFCC units, this is about 32 mmol/mol. Formula path: 5.7 GMI → 99.92 mg/dL → 5.11% A1C.
6.1 GMI to A1C
6.1 GMI is about 5.69% A1C. On this page’s formula set, that corresponds to an estimated mean glucose of about 116.64 mg/dL.
In IFCC units, this is about 39 mmol/mol. Formula path: 6.1 GMI → 116.64 mg/dL → 5.69% A1C.
5.8 GMI to A1C
5.8 GMI is about 5.25% A1C. On this page’s formula set, that corresponds to an estimated mean glucose of about 104.10 mg/dL.
In IFCC units, this is about 34 mmol/mol. Formula path: 5.8 GMI → 104.10 mg/dL → 5.25% A1C.
6.2 GMI to A1C
6.2 GMI is about 5.84% A1C. On this page’s formula set, that corresponds to an estimated mean glucose of about 120.82 mg/dL.
In IFCC units, this is about 40 mmol/mol. Formula path: 6.2 GMI → 120.82 mg/dL → 5.84% A1C.
5.9 GMI to A1C
5.9 GMI is about 5.40% A1C. On this page’s formula set, that corresponds to an estimated mean glucose of about 108.28 mg/dL.
In IFCC units, this is about 36 mmol/mol. Formula path: 5.9 GMI → 108.28 mg/dL → 5.40% A1C.
6.8 GMI to A1C
6.8 GMI is about 6.71% A1C. On this page’s formula set, that corresponds to an estimated mean glucose of about 145.90 mg/dL.
In IFCC units, this is about 50 mmol/mol. Formula path: 6.8 GMI → 145.90 mg/dL → 6.71% A1C.
6.4 GMI to A1C
6.4 GMI is about 6.13% A1C. On this page’s formula set, that corresponds to an estimated mean glucose of about 129.18 mg/dL.
In IFCC units, this is about 43 mmol/mol. Formula path: 6.4 GMI → 129.18 mg/dL → 6.13% A1C.
6.6 GMI to A1C
6.6 GMI is about 6.42% A1C. On this page’s formula set, that corresponds to an estimated mean glucose of about 137.54 mg/dL.
In IFCC units, this is about 47 mmol/mol. Formula path: 6.6 GMI → 137.54 mg/dL → 6.42% A1C.
These are estimates based on the GMI and ADAG formulas used on this page. GMI and lab A1C can differ because of red blood cell lifespan, hemoglobin variants, pregnancy, kidney disease, recent blood loss, and related factors.
What Is GMI?
GMI (Glucose Management Indicator) replaced the older “estimated A1C” (eA1C) label on CGM reports in 2018, after the FDA raised concerns that the term implied diagnostic equivalence to a laboratory blood test. GMI is generated automatically by major CGM platforms: Dexcom Clarity (G6/G7), LibreView (FreeStyle Libre 2/3), Medtronic CareLink, and Tandem t:connect. A minimum of 10 days of sensor data is required; 14 or more days yields more stable estimates of glucose variability and time below range. A sensor wear of at least 70% over the reporting period is generally recommended for a reliable GMI.
Why GMI and Lab A1C Often Disagree
In the original Bergenstal 2018 validation study, 28% of participants had a GMI that differed from their concurrent lab A1C by 0.5% or more. GMI reflects only the CGM measurement window (as short as 10 days) while HbA1c reflects 60 to 90 days of red blood cell glycation. Additional physiologic factors drive discordance:
| Factor | Effect on Lab A1C | Effect on GMI |
|---|---|---|
| Shortened RBC lifespan (sickle cell, hemolysis) | Falsely low A1C | None |
| Prolonged RBC lifespan (iron deficiency anemia) | Falsely high A1C | None |
| Hemoglobin variants (HbS, HbC, HbE, HbF) | Assay-dependent; raises or lowers A1C | None |
| Recent blood transfusion | Falsely low A1C (new RBCs have little glycation) | None |
| Pregnancy (faster RBC turnover) | Reduces A1C by roughly 0.5 to 1.0% | None |
| Chronic kidney disease (CKD) | Falsely low A1C due to shortened RBC survival | Unaffected by RBC factors |
| Measurement time window | Reflects prior 60 to 90 days of glycation | Reflects only the CGM wear period (10+ days) |
| When GMI is consistently higher than lab A1C, shortened RBC lifespan is the most common cause. When GMI is consistently lower, iron deficiency anemia or hemoglobin variants are common drivers. | ||
A1C and GMI Targets with Mean Glucose Equivalents
ADA 2024 Standards of Care set an A1C goal of less than 7.0% for most non-pregnant adults with diabetes, with individualized targets ranging from 6.0% to 8.0% based on hypoglycemia risk, life expectancy, and patient preference. The ADAG and GMI formulas imply different mean glucose values for the same A1C target, with the gap widening at the extremes of glycemic control:
| A1C / GMI Target | Mean Glucose via ADAG (mg/dL) | Mean Glucose via GMI Formula (mg/dL) | Clinical Context |
|---|---|---|---|
| 6.0% | 126 | 112 | Very stringent; pregnancy, recent T1D diagnosis |
| 6.5% | 140 | 133 | Stringent; low hypoglycemia risk |
| 7.0% (ADA standard) | 154 | 154 | ADA general adult target (formulas converge here) |
| 7.5% | 169 | 175 | Moderate; balancing control with hypoglycemia risk |
| 8.0% | 183 | 196 | Relaxed target; elderly or high hypoglycemia risk |
| 9.0% | 212 | 238 | Action threshold; intensification typically indicated |
| ADAG: mg/dL = 28.7 x A1C(%) – 46.7. GMI formula: mg/dL = (A1C(%) – 3.31) / 0.02392. Divergence grows at the extremes because the two regression lines have different slopes. At 6.5%, the GMI formula implies a 7 mg/dL lower mean glucose requirement than ADAG; at 9.0%, the gap widens to 26 mg/dL. | |||
