Mean Blood Glucose to A1C: ADAG Formula, GMI, and Why They Disagree (2026)

Compare ADAG-estimated A1C and GMI from mean glucose, with checked examples and tables. Understand sampling limitations and differences from laboratory A1C.

A mean glucose of 148 mg/dL gives an ADAG-estimated A1C of 6.78% and a GMI of 6.85%. Neither calculation determines your individual laboratory A1C. Different equations, sampling periods, and biological factors can explain a mismatch without either measured value necessarily being wrong.

The A1C Calculator converts average glucose to an ADAG estimate, or an entered A1C to estimated average glucose (eAG). The comparison below adds the separate GMI equation so you can understand the distinction; GMI is not an additional calculator input mode.

What Mean Glucose Represents#

A mean averages the recorded glucose values. Its usefulness depends on what the readings cover: a regular CGM series includes times a fasting-only fingerstick log misses. Check the date range, gaps, and sampling pattern before comparing reports. An average from a recent fortnight and a lab A1C reflecting roughly three months cover different periods.

The same mean can hide very different patterns. Readings around 100–160 mg/dL and swings between 60 and 220 mg/dL can both average 140 mg/dL. Review glucose variability and time in range alongside the mean with your care team.

ADAG: Mean Glucose to Estimated A1C#

The ADAG study, Nathan and colleagues (2008) established an equation relating A1C to estimated average glucose. It included 507 participants and combined CGM with frequent fingerstick measurements over three months. Reversing its equation gives:

Estimated A1C (%) = (Mean Glucose mg/dL + 46.7) / 28.7
eAG (mg/dL) = (28.7 × A1C) - 46.7

At 148 mg/dL: (148 + 46.7) / 28.7 = 6.78%. In the other direction, a measured A1C of 7.0% gives (28.7 × 7.0) - 46.7 = 154.2 mg/dL.

The calculator also accepts mmol/L, multiplying by 18.018 before using the mg/dL equation. These are estimates from a study population; they do not replace testing.

Values are rounded to one decimal place. These glucose averages must not be interpreted as fasting-glucose diagnostic cutoffs.

GMI: A Separate Equation for CGM Data#

The 2018 GMI study, Bergenstal and colleagues, developed a CGM-based equation and terminology that distinguishes its output from a laboratory A1C measurement:

GMI (%) = 3.31 + (0.02392 × Mean CGM Glucose mg/dL)

At 148 mg/dL: 3.31 + (0.02392 × 148) = 6.85%. The coefficients come from a different fitted relationship, so GMI is not simply the ADAG formula in another form. The paper discusses individual differences between GMI and measured A1C; a universal agreement guarantee would be misleading.

Differences use unrounded calculations, then round to two decimals. The equations intersect at about 154.06 mg/dL; at 154 they agree to the precision shown. Similar values near that point are a property of the equations, not proof that either matches your lab A1C.

GMI summarizes the mean CGM glucose on an A1C-like scale. It cannot be used to convert a CGM report into an individual's actual lab result or to apply diagnostic A1C thresholds.

Reasons the Lab Result Can Differ#

Different time windows. NIDDK's A1C explanation describes a result reflecting about three months, with recent glucose contributing more strongly. A simple average over another interval does not reproduce that weighting.

Hemoglobin and red cells. NGSP's interference guidance distinguishes lower A1C associated with shortened red-cell survival from higher A1C associated with iron deficiency anemia. Hemoglobin variants can affect some assays; the direction depends on the variant and method. Transfusion and kidney disease can complicate interpretation.

The underlying glucose data. Missing readings, checks concentrated at certain times, or sensor problems can change a reported mean. A disagreement can have more than one cause. It cannot diagnose iron deficiency or a hemoglobin disorder, and a single test recommendation cannot resolve every mismatch.

Bring the lab date and the glucose report to your clinician if the mismatch persists or seems unexpected. They can review the data and decide whether the assay or another condition needs assessment. Treatment decisions can draw on lab tests, CGM patterns, symptoms, and other clinical information; neither this estimate nor an isolated threshold determines medication dosing.

Using the Calculator Alongside a Lab Report#

1. Identify the glucose report's period and whether it represents more than fasting readings.
2. Enter the average and correct unit in the A1C Calculator to get the ADAG estimate.
3. Record that the result is calculated and keep it separate from any measured laboratory A1C or CGM-reported GMI.
4. Use the reverse mode if you want to express an entered A1C as eAG. This does not recover the individual readings behind a lab result.

For the unit conversion steps and eAG reference table, see the blood-glucose conversion guide.