CGM · GMI
GMI calculator, mean glucose to A1C estimate.
Enter the mean glucose from your CGM report and see the glucose management indicator (GMI), an estimate of A1C, in percent and mmol/mol. Optionally compare it with a lab A1C.
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How GMI is calculated
The glucose management indicator (GMI) turns the mean glucose from a continuous glucose monitor (CGM) into an estimate of A1C. The published equations are GMI (%) = 3.31 + 0.02392 × mean glucose in mg/dL and GMI (mmol/mol) = 12.71 + 4.70587 × mean glucose in mmol/L. The authors say 10 to 14 days of CGM data gives a representative mean, and that 14 days or more is better for time in hypoglycemia and variability.
Worked example. A mean glucose of 154 mg/dL: 3.31 + 0.02392 × 154 = 7.0%. In mmol/L that mean is 154 ÷ 18.016 = 8.5 mmol/L, and 12.71 + 4.70587 × 8.548 = 52.9 mmol/mol. With a lab A1C of 7.5%, the lab value is 0.5 points above the GMI.
Why GMI and lab A1C differ. The paper says a difference between laboratory A1C and GMI is not unexpected and may be important to consider; the same GMI can go with different measured A1C values for different people, and the gap can be larger during rapid changes. This page uses the 2018 equation. A 2026 Diabetologia paper titled “Updated glucose management indicator (GMI) better aligns with HbA1c than current GMI” proposes a newer one, so ask your care team which figure they use.
Frequently asked questions
What is the GMI formula?
GMI in percent is 3.31 + 0.02392 times the mean glucose in mg/dL. For a mean of 154 mg/dL that is 3.31 + 0.02392 x 154 = 7.0 percent.
Is GMI the same as A1C?
No. GMI estimates A1C from CGM mean glucose, and the two often differ for the same person. A difference between lab A1C and GMI is not unexpected and is worth discussing with a care team.
How many days of CGM data do I need?
The authors say 10 to 14 days gives a representative mean glucose, and 14 days or more is better for time in hypoglycemia and glucose variability.
Does the formula work in mmol/L?
Yes. GMI in mmol/mol is 12.71 + 4.70587 times the mean in mmol/L. This page also converts mmol/L to mg/dL for the percent formula.
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Educational worked example, not medical advice
This page shows arithmetic and published rules of thumb for learning. It is not a dosing instruction, a diagnosis or a treatment plan, and it does not know your insulin, your body or your health history. Never change insulin or medicine because of a number on this page. Confirm every setting and target with your diabetes care team. If you have symptoms of a very low or very high glucose, follow your care plan; in an emergency call your local emergency number.