GMI Calculator (Glucose Management Indicator from CGM Average)
Estimate the glucose management indicator (GMI), the A1C-equivalent of a continuous glucose monitor average, from the mean glucose in mg/dL or mmol/L using the Bergenstal 2018 formulas.
Last updated: October 2026
GMI
6.99 %
This calculator gives an estimate for general information only. It is not medical advice and does not replace a doctor, pharmacist or other qualified health professional. Always check with them before acting on the result, especially for medication doses, children, pregnancy or a medical condition, and follow the guidelines of your local health authority.
Formula below · 3 sources (pmc.ncbi.nlm.nih.gov, niddk.nih.gov, diabetes.org) · Updated Oct 2026
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About this calculator
The glucose management indicator (GMI) is the HbA1c you would expect from the mean glucose measured by a continuous glucose monitor. It replaced the older "estimated A1C" on CGM reports in 2018 to make clear that it is calculated from glucose, not measured in blood. The formulas, from Bergenstal and colleagues (Diabetes Care 2018), were fitted to 528 people with diabetes:
GMI (%) = 3.31 + 0.02392 × mean glucose (mg/dL) GMI (mmol/mol) = 12.71 + 4.70587 × mean glucose (mmol/L)
When you enter mmol/L the calculator uses the mmol/mol formula and converts to percent with the NGSP–IFCC master equation; both routes give the same answer to within 0.01%.
How it is used. The international consensus on CGM reporting (Battelino 2019) lists GMI with time in range (70-180 mg/dL, goal above 70%), time below range (goal below 4%) and the coefficient of variation (goal 36% or less) on the ambulatory glucose profile report, based on at least 14 days of data with 70% or more sensor wear. GMI lets the CGM average be compared with the HbA1c goal the person already knows.
Why GMI and HbA1c differ. HbA1c reflects about three months of glucose weighted towards the most recent month and also depends on how long red cells live and how fast haemoglobin glycates, which varies between people. GMI reflects only the sensor window. In the derivation study the two were within 0.5% for about 70% of people, and differences of 1% or more occur; a persistent gap is itself useful, because it reveals a person whose HbA1c runs high or low for their actual glucose. The older ADAG equation (eAG) describes the opposite direction, average glucose from HbA1c, and gives a slightly different A1C for the same mean; the tile shows it for comparison.
GMI is an estimate for information and for discussion with a diabetes care team; it does not replace the laboratory HbA1c.
CGM mean glucose and the corresponding GMI
Scroll sideways to see all columns →
| Mean glucose (mg/dL) | Mean glucose (mmol/L) | GMI (%) | GMI (mmol/mol) |
|---|---|---|---|
| 100 | 5.6 | 5.7 | 39 |
| 125 | 6.9 | 6.3 | 45 |
| 154 | 8.6 | 7 | 53 |
| 175 | 9.7 | 7.5 | 58 |
| 200 | 11.1 | 8.1 | 65 |
| 225 | 12.5 | 8.7 | 71 |
| 250 | 13.9 | 9.3 | 78 |
How to use
Read the mean (average) glucose from the CGM app or the ambulatory glucose profile report for at least 14 days, and enter it with its unit. Example (the defaults): mean glucose 154 mg/dL. GMI = 3.31 + 0.02392 × 154 = 3.31 + 3.68 = 6.99, shown as 7.0%, which is (6.99 − 2.15) × 10.929 = 53 mmol/mol. The ADAG equation gives (154 + 46.7) ÷ 28.7 = 7.0% for the same mean, so the two agree here. Example in mmol/L: mean glucose 10.0 mmol/L. GMI = 12.71 + 4.70587 × 10.0 = 59.8 mmol/mol, which converts to 59.8 ÷ 10.929 + 2.15 = 7.6%. In mg/dL the same mean is 180 and 3.31 + 0.02392 × 180 = 7.6%.
Frequently asked questions
What is a good GMI?
The same as a good HbA1c for you. Below 7% (53 mmol/mol) is the usual goal for most adults with diabetes, below 6.5% for some younger people without hypoglycaemia, and below 8% for older or frail adults. A mean glucose of about 154 mg/dL (8.6 mmol/L) corresponds to a GMI of 7%.
Why is my GMI different from my lab A1C?
Because they measure different things over different windows. HbA1c depends on red-cell lifespan and individual glycation rate as well as glucose, and covers about three months; GMI is calculated only from the sensor glucose in the period you choose. A difference of up to 0.5% is common and larger gaps occur. A GMI that is consistently lower than HbA1c suggests the person glycates faster than average (or the sensor reads low), and vice versa.
How many days of CGM data do I need?
At least 14 days with the sensor worn 70% of the time or more, according to the international consensus on CGM metrics. Shorter periods are more affected by a few unusual days; 30 or 90 days give a steadier figure.
Is GMI the same as estimated A1C (eA1C)?
It is the replacement for it. CGM reports used to label the figure "estimated A1C", which caused confusion when it did not match the laboratory result. GMI uses an updated formula and a name that says what it is: an indicator from glucose data.
Can I use a glucose meter average instead of a CGM?
The formula was derived from CGM data, which samples glucose every 5-15 minutes including overnight. A meter average from a few readings a day, often before meals, is usually lower than the true mean and would give a GMI that is too low. Use it only as a rough guide.
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Sources & references
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