Guide
GMI vs A1C: what your CGM number means
Continuous glucose monitor (CGM) reports show a GMI, an A1C-like number calculated from your sensor average. Here is the formula, a chart of common values and why it does not always match your lab A1C.
What is GMI?
The Glucose Management Indicator is an estimate of what your A1C would be, calculated from the average glucose measured by a CGM. An expert group (Bergenstal et al.) introduced it in 2018 to replace the older term “estimated A1C”, precisely because the number often differs from a lab test. It is shown in reports such as Dexcom Clarity and LibreView.
The GMI formula
GMI (%) = 3.31 + 0.02392 × mean glucose (mg/dL)
With the mean glucose in mmol/L the formula is GMI (%) = 3.31 + 0.43056 × mean glucose. For example, a mean of 154 mg/dL gives 3.31 + 0.02392 × 154 ≈ 7.0%.
To go from GMI to the mean glucose, reverse it: mean glucose (mg/dL) = (GMI − 3.31) ÷ 0.02392. A GMI of 7.0% corresponds to a mean of about 154 mg/dL. GMI uses the same units as A1C, so you can convert it to mmol/mol with the same equation: (GMI − 2.15) × 10.929, which makes 7.0% about 53 mmol/mol.
GMI chart
| Mean glucose (mg/dL) | Mean glucose (mmol/L) | GMI (%) | GMI (mmol/mol) | A1C from eAG formula (%) |
|---|---|---|---|---|
| 100 | 5.6 | 5.7 | 39 | 5.1 |
| 115 | 6.4 | 6.1 | 43 | 5.6 |
| 125 | 6.9 | 6.3 | 45 | 6.0 |
| 140 | 7.8 | 6.7 | 49 | 6.5 |
| 150 | 8.3 | 6.9 | 52 | 6.9 |
| 155 | 8.6 | 7.0 | 53 | 7.0 |
| 170 | 9.4 | 7.4 | 57 | 7.6 |
| 180 | 10.0 | 7.6 | 60 | 7.9 |
| 200 | 11.1 | 8.1 | 65 | 8.6 |
| 225 | 12.5 | 8.7 | 71 | 9.5 |
| 250 | 13.9 | 9.3 | 78 | 10.3 |
| 275 | 15.3 | 9.9 | 85 | 11.2 |
| 300 | 16.7 | 10.5 | 91 | 12.1 |
The two formulas agree around 7%, but they come from different studies and drift apart at higher averages. GMI was derived from CGM studies in people with type 1 and type 2 diabetes; the eAG formula comes from the ADAG study, which also included people without diabetes.
Why GMI and lab A1C are not the same
In the original study, GMI and lab A1C differed by 0.5 percentage point or more in about half of people, and by 1 point or more in about one in ten. That does not mean either one is wrong. A1C depends on how long your red blood cells live and how readily glucose attaches to hemoglobin, which varies between people. GMI depends only on the sensor data.
Knowing your personal gap is useful: if your lab A1C is consistently 0.4 point above your GMI, you and your doctor can take that into account when setting goals.
Getting a reliable GMI
- Use at least 14 days of data, ideally the whole period before your lab test.
- Aim for at least 70% sensor wear time over that period.
- Compare GMI and lab A1C that cover the same weeks.
GMI, time in range and A1C together
The international consensus recommends reporting time in range (70–180 mg/dL, 3.9–10.0 mmol/L) alongside GMI. For most adults with type 1 or type 2 diabetes the goal is more than 70% of the time in range, which roughly corresponds to an A1C of about 7%. Time in range shows highs and lows that an average can hide.
Common questions
Is GMI the same as A1C?
No. GMI is calculated from CGM glucose; A1C is a blood test. They are often close but can differ by half a point or more for physiological reasons.
My GMI is lower than my A1C. Which one is right?
Both can be right. Share both numbers with your care team; a steady gap is common and helps them read future results.
This page is for information only and is not medical advice.
Sources
- Bergenstal RM et al. Glucose Management Indicator (GMI): a new term for estimating A1C from continuous glucose monitoring. Diabetes Care 2018;41(11):2275–2280.
- Battelino T et al. Clinical targets for continuous glucose monitoring data interpretation: recommendations from the international consensus on time in range. Diabetes Care 2019;42(8):1593–1603.
- Nathan DM et al. Translating the A1C assay into estimated average glucose values. Diabetes Care 2008;31(8):1473–1478.