Estimated average glucose (ADAG): how to convert HbA1c to mg/dL and when the conversion fails
What estimated average glucose is and where the equation comes from
Estimated average glucose (eAG) translates glycated hemoglobin (HbA1c) into the same unit as everyday blood glucose, mg/dL. HbA1c reflects the exposure of hemoglobin to glucose over about three months, but it is expressed as a percentage, which makes comparison with fingerstick glucose readings difficult.
The equation comes from the ADAG (A1c-Derived Average Glucose) study, published by Nathan and colleagues in Diabetes Care in 2008. The study analyzed 507 adults: 268 with type 1 diabetes, 159 with type 2 and 80 without diabetes. Each participant had continuous glucose monitoring and seven-point fingerstick glucose profiles for three months, with about 2,700 measurements per person.
The linear relationship between HbA1c and average glucose had an R² of 0.84. The authors concluded that HbA1c can be expressed as eAG for most people with type 1 and type 2 diabetes. To study insulin resistance, an aspect that HbA1c does not show, there is the HOMA-IR index.
How to calculate estimated average glucose in the calculator
The calculator asks for a single input: glycated hemoglobin (HbA1c) as a percentage, the NGSP/DCCT unit that appears on most Brazilian laboratory reports. The field accepts values from 4% to 14%; outside that range, an error message appears.
The tool's step-by-step process is as follows:
- Receives the HbA1c entered and checks the range.
- Calculates eAG in mg/dL: 28.7 × HbA1c − 46.7, rounded to a whole number.
- Calculates eAG in mmol/L: 1.5944 × HbA1c − 2.5944, with one decimal place.
- Shows a note on individual variation around the average.
The table published by the ADAG study runs from 5% to 12%. If the HbA1c entered is between 4% and 5% or between 12% and 14%, the tool warns that the value is an extrapolation of the equation.
The calculator does not classify average glucose: it only converts. The page separately shows the table of HbA1c criteria, which refers to HbA1c. If the report gives HbA1c in mmol/mol (IFCC unit), convert it to % before entering it, as explained in the units section.
2008 ADAG formula: equation in mg/dL and in mmol/L
The equation used by the calculator is the linear regression published in the ADAG study (Nathan and colleagues, 2008).
eAG (mg/dL) = 28.7 × HbA1c (%) − 46.7
eAG (mmol/L) = 1.5944 × HbA1c (%) − 2.5944
- eAG: estimated average glucose for the previous three months or so.
- HbA1c: glycated hemoglobin as a percentage, standardized by the NGSP and traceable to the DCCT.
- 28.7: slope of the line, that is, how many mg/dL average glucose rises for each percentage point of HbA1c.
- −46.7: intercept of the line.
The calculator shows both versions. The mmol/L equation appears in the article's table; in the text, the authors round it to 1.59 × HbA1c − 2.59.
A practical consequence of the slope: each 1% difference in HbA1c corresponds to about a 29 mg/dL difference in estimated average glucose.
Worked example: HbA1c of 7% and 6.5%
The examples below are hypothetical and serve only to check the calculation.
HbA1c of 7.0%
- 28.7 × 7 = 200.9.
- 200.9 − 46.7 = 154.2.
- Rounded: eAG = 154 mg/dL.
The ADAG study published the same value for 7%, with 95% limits between 123 and 185 mg/dL. In mmol/L: 1.5944 × 7 − 2.5944 = 8.6 mmol/L, also matching the article's table.
HbA1c of 6.5%
- 28.7 × 6.5 = 186.55.
- 186.55 − 46.7 = 139.85.
- Rounded: eAG = 140 mg/dL.
The calculator returns 154 mg/dL (8.6 mmol/L) and 140 mg/dL (7.8 mmol/L) for these data. The 123 to 185 mg/dL interval for the same HbA1c of 7% shows the size of the variation between people: eAG is a group average, not the exact glucose of the person tested.
How to understand estimated average glucose
Estimated average glucose is a way of communicating HbA1c, not a new test. It adds no information beyond HbA1c itself; it only changes the scale.
Diagnostic criteria apply to HbA1c, not to eAG. According to the guideline of the Sociedade Brasileira de Diabetes (Brazilian Diabetes Society; 2026 edition, 2024 revision), HbA1c is considered normal below 5.7%, indicates prediabetes between 5.7% and 6.4% and diabetes from 6.5%, a criterion that requires confirmation according to the guideline itself.
| HbA1c (%) | eAG by ADAG (mg/dL) | 95% limits in the study (mg/dL) |
|---|---|---|
| 6 | 126 | 100 to 152 |
| 7 | 154 | 123 to 185 |
| 8 | 183 | 147 to 217 |
| 9 | 212 | 170 to 249 |
| 10 | 240 | 193 to 282 |
When eAG differs greatly from fingerstick readings or continuous monitoring, the discrepancy is useful information: it may indicate a condition that alters HbA1c. Decisions about targets and management belong to the professional caring for the patient.
Common errors and conversion between % and mmol/mol
The most common error is entering HbA1c in mmol/mol, the IFCC unit, in the field that expects a percentage. A value of 53 mmol/mol entered as 53% would produce an absurd glucose value.
The official NGSP conversion uses the master equation:
HbA1c (%) = 0.09148 × HbA1c (mmol/mol) + 2.152
Thus, 53 mmol/mol corresponds to 7.0%, a value the NGSP table also lists.
Other frequent errors:
- Comparing eAG with a single fasting glucose: eAG includes postprandial and nighttime periods, so it usually differs from the fasting value.
- Using eAG for diagnosis: diagnostic cutoffs apply to HbA1c.
- Entering values outside the range: the calculator accepts HbA1c from 4% to 14% and shows an error message outside it, which also blocks most values entered in mmol/mol.
Checklist: is HbA1c in %? Is the laboratory method NGSP-certified? Is there a condition that alters red blood cell lifespan? Was the result rounded to a whole number, as in the tool?
When HbA1c and estimated average glucose fail
The conversion inherits all the limitations of HbA1c and adds the scatter of the equation. The guideline of the Sociedade Brasileira de Diabetes lists situations prone to HbA1c discrepancies:
- hemoglobin variants and hemoglobinopathies;
- pregnancy and the postpartum period;
- anemias and blood transfusions;
- use of antiretrovirals;
- chronic kidney failure;
- use of recombinant erythropoietin.
The ADAG study itself excluded children, pregnant women and people with anemia, hemoglobinopathies, high red cell turnover, blood loss or transfusion, or chronic kidney or liver disease, and included adults aged 18 to 70. Outside that profile, the equation was not tested. In kidney disease, it is worth checking the glomerular filtration rate by CKD-EPI before relying on the conversion.
Compared with continuous glucose monitoring, eAG does not show variability, hypoglycemia or time in range. Compared with fasting glucose, it integrates a long period, but does not capture changes in recent weeks as quickly. Beta-cell function, which explains part of these variations, can be estimated with the HOMA-B index.
Frequently asked questions about estimated average glucose from HbA1c
What average glucose corresponds to an HbA1c of 7%?
About 154 mg/dL by the ADAG equation (28.7 × 7 − 46.7). In the original study, 95% of people with an HbA1c of 7% had an average glucose between 123 and 185 mg/dL.
Can estimated average glucose be used to diagnose diabetes?
No. Diagnostic criteria apply to HbA1c itself (diabetes from 6.5%, according to the Sociedade Brasileira de Diabetes, with confirmation). eAG only translates HbA1c into mg/dL.
My report gives HbA1c in mmol/mol. How do I use the calculator?
Convert it to a percentage with the NGSP equation: % = 0.09148 × mmol/mol + 2.152. For example, 53 mmol/mol is equivalent to 7.0%.
Why doesn't estimated average glucose match my fingerstick readings?
Because eAG is a group average and fingerstick measurements capture isolated moments. Anemias, hemoglobinopathies, kidney disease and pregnancy also alter HbA1c. Large differences should be discussed with the professional following the case.
Does the calculator interpret the result?
No. It only converts HbA1c into estimated average glucose, in mg/dL (whole number) and in mmol/L (one decimal place). Outside the 5% to 12% range of the ADAG study table, it warns that the value is extrapolated. The criteria table shown on the page refers to HbA1c.
References
- Nathan DM, Kuenen J, Borg R, Zheng H, Schoenfeld D, Heine RJ; A1c-Derived Average Glucose (ADAG) Study Group. Translating the A1C assay into estimated average glucose values. Diabetes Care. 2008;31(8):1473-8. View
- Rodacki M, Cobas RA, Zajdenverg L, et al. Diagnóstico de diabetes mellitus. Diretriz Oficial da Sociedade Brasileira de Diabetes. 2026 edition (revised July 9, 2024). View
- National Glycohemoglobin Standardization Program (NGSP). IFCC Standardization: IFCC and NGSP. View
Updated . A support tool for educational purposes; it does not replace clinical assessment.