The Glucose Management Indicator (GMI) is an estimate of your lab hemoglobin A1C calculated from 14 or more days of continuous glucose monitor (CGM) data. The formula is GMI (%) = 3.31 + 0.02392 × mean glucose in mg/dL, published by Bergenstal and colleagues in Diabetes Care in 2018. GMI and lab A1C often differ by up to 0.5 percentage points because they measure different things — use them together, not interchangeably.
The GMI Formula and How to Use It
The formula takes your CGM-reported average glucose (from at least 14 days of data, with the sensor active for at least 70% of that time) and converts it to an estimated A1C.
GMI (%) = 3.31 + 0.02392 × mean glucose (mg/dL)
Example: if your Dexcom G7, FreeStyle Libre 3 Plus, or Stelo reports a 14-day average of 154 mg/dL, your GMI is 3.31 + (0.02392 × 154) = 6.99%, typically rounded to 7.0%.
GMI to A1C Conversion Table
| Mean Glucose (mg/dL) | Mean Glucose (mmol/L) | GMI (%) |
|---|---|---|
| 100 | 5.6 | 5.7 |
| 120 | 6.7 | 6.2 |
| 140 | 7.8 | 6.7 |
| 154 | 8.6 | 7.0 |
| 170 | 9.4 | 7.4 |
| 183 | 10.2 | 7.7 |
| 200 | 11.1 | 8.1 |
| 230 | 12.8 | 8.8 |
| 269 | 14.9 | 9.7 |
Most CGM reporting platforms — Dexcom Clarity, LibreView, Tidepool, Glooko — calculate GMI automatically once you have at least 14 days of data with 70%+ sensor wear. Ask your clinician or diabetes educator for help reading the report if the number does not appear obvious.
Why GMI and Lab A1C Disagree
GMI and A1C often move together, but they can diverge meaningfully. The two tests measure different things:
- Lab A1C measures the percentage of hemoglobin that has glucose bound to it. Because red blood cells live about 120 days, A1C reflects average glucose over roughly 3 months — but it is heavily weighted toward the most recent month.
- GMI reflects only the time window of your CGM data, typically 14-90 days. It ignores red blood cell biology entirely.
Conditions that can cause A1C to read higher or lower than GMI include:
- Iron-deficiency anemia — longer red cell lifespan, higher A1C.
- Hemolytic anemia or recent blood loss — shorter red cell lifespan, lower A1C.
- Pregnancy — faster red cell turnover, typically lower A1C.
- Chronic kidney disease with erythropoietin use — falsely lowered A1C.
- Hemoglobinopathies (sickle cell trait, thalassemia, HbC) — unreliable A1C regardless of direction.
- Recent blood transfusion — unpredictable effect.
If your GMI consistently differs from lab A1C by more than 0.5 percentage points, bring it up with your clinician. One of the two numbers may be biased by a red blood cell condition, and the treatment plan should be driven by the more reliable measure. For a deeper primer on A1C, see our guide to A1C levels.
How to Get a Reliable GMI
A high-quality GMI requires:
- At least 14 consecutive days of CGM wear — ideally closer to 30 days.
- Sensor data capture of 70% or higher during that window.
- No prolonged sensor failures or significant calibration errors.
- Routine activity and eating patterns — vacations, illness, or unusual stress can skew the average.
If you miss doses or remove the sensor for multiple showers or airport security checks, the GMI may underrepresent your real glucose exposure. Most CGM apps flag incomplete data automatically.
Using GMI Alongside Time in Range
GMI is only one number in a modern diabetes report. The international CGM consensus recommends reviewing five metrics together:
- Time in range (70-180 mg/dL): Target is 70% or more for most adults with type 1 or type 2 diabetes.
- Time below range (under 70 mg/dL): Target under 4%, with less than 1% below 54 mg/dL.
- Time above range (over 180 mg/dL): Target under 25%, with less than 5% above 250 mg/dL.
- Mean glucose: Drives GMI directly.
- Glucose variability (CV%): Target under 36%.
Two people can have the same GMI of 7.0% with very different glucose profiles — one stable around 154 mg/dL, the other swinging between 60 and 250 mg/dL. Looking only at GMI would miss that difference, while time in range and CV would catch it immediately.
Practical Examples
Case 1: Matched GMI and A1C. A 42-year-old with type 2 diabetes has a 30-day GMI of 6.6% and a lab A1C of 6.7%. Treatment continues unchanged; both numbers agree.
Case 2: GMI lower than A1C. A 28-year-old with type 1 diabetes has a GMI of 7.1% but lab A1C of 8.0%. Blood work reveals mild iron deficiency — iron repletion brings A1C closer to GMI, confirming the discrepancy was red cell biology, not glucose control.
Case 3: GMI higher than A1C. A 65-year-old with chronic kidney disease has GMI 7.8% and lab A1C 6.9%. Erythropoietin therapy is accelerating red cell turnover. GMI is more reliable for this person’s treatment decisions.
Can I Calculate GMI Without a CGM?
No. GMI requires mean glucose from continuous or flash glucose monitoring. Fingerstick averages are too sparse and are biased toward waking hours, especially pre-meal testing. The old “eAG” (estimated average glucose) from a lab A1C is different — that goes the other direction, converting A1C back to glucose. For broader background on CGM options, see our treatment and monitoring hub.
The Bottom Line
GMI is a useful, easy-to-interpret estimate of A1C derived from CGM data, calculated with the formula GMI (%) = 3.31 + 0.02392 × mean glucose. Expect some disagreement with lab A1C — up to 0.5 percentage points in most people, more if red blood cell biology is altered. Use GMI alongside time in range, glucose variability, and periodic lab A1C testing, and discuss any large discrepancies with your clinician before changing therapy.
Sources
- Bergenstal RM, Beck RW, Close KL, et al. Glucose Management Indicator (GMI): A New Term for Estimating A1C From Continuous Glucose Monitoring. Diabetes Care. 2018;41(11):2275-2280.
- American Diabetes Association. Standards of Care in Diabetes 2025 — Glycemic Goals.
- Battelino T, et al. Clinical Targets for CGM Data Interpretation. Diabetes Care. 2019;42(8):1593-1603.
- National Institute of Diabetes and Digestive and Kidney Diseases. The A1C Test and Diabetes.
