GMI on the FreeStyle Libre 3 stands for Glucose Management Indicator. It’s a number displayed as a percentage that estimates what your lab A1c result would be, based on the average glucose readings your sensor has collected. If your GMI reads 7.0%, for instance, it suggests your A1c would land around 7.0% if your glucose patterns stayed the same over the next month or so.
Where to Find GMI on the Libre 3 App
GMI lives inside the reports section of the Libre app. To get there, tap the Insights menu icon at the bottom of your screen, then tap the Reports tab at the top. You’ll see a list of reports including Time in Ranges, Average Glucose, Low Glucose Events, Daily Patterns, and GMI. Tap on GMI to expand the report and see your current value.
The number requires at least 14 days of sensor data, with the sensor active at least 70% of the time, before it’s considered reliable. If you’ve just started wearing a Libre 3, give it a couple of weeks before putting much stock in the GMI reading.
How GMI Is Calculated
The calculation is straightforward. Your sensor tracks glucose readings around the clock, producing an average over the reporting period. That average gets plugged into a formula developed by the Jaeb Center for Health Research:
GMI (%) = 3.31 + (0.02392 × mean glucose in mg/dL)
So if your average glucose over the past 14 days is 154 mg/dL, the math works out to a GMI of roughly 7.0%. A higher average glucose pushes the number up; a lower average brings it down. The formula was built from population-level data comparing thousands of people’s CGM readings against their lab A1c results.
GMI vs. Lab A1c: They Often Don’t Match
This is the most important thing to understand about GMI. It is not an A1c test. It’s an estimate of what your A1c might be, and for many people, the two numbers don’t line up. Studies have consistently found that 26% to 68% of individuals show a clinically meaningful gap of 0.5 percentage points or more between their GMI and their actual lab A1c.
There are real biological reasons for this mismatch. A1c measures how much sugar has attached to hemoglobin in your red blood cells over the past two to three months. That process depends on factors GMI can’t account for: how long your red blood cells live (which varies from person to person), how readily your hemoglobin binds to glucose, and genetic factors that researchers still don’t fully understand. Someone with shorter-lived red blood cells, for example, will tend to have a lower A1c than their average glucose would predict, making their GMI appear higher than their lab result.
The discrepancy can be especially pronounced at the extremes. If your glucose control is very tight or very poor, the gap between GMI and lab A1c tends to widen.
What GMI Is Actually Useful For
Despite its limitations as an A1c substitute, GMI serves a practical purpose: it gives you a real-time trend between lab visits. Your A1c only updates every two to three months when you get blood drawn. GMI updates continuously as your sensor collects data, so you can see the effects of dietary changes, medication adjustments, or exercise patterns weeks before your next lab appointment.
Some clinicians consider GMI more reflective of day-to-day glucose management than A1c for making treatment decisions. The reasoning is that GMI captures what’s actually happening with your blood sugar right now, while A1c is a backward-looking average that can be skewed by the biological factors mentioned above. The National Institute of Diabetes and Digestive and Kidney Diseases has noted that GMI “can be used to guide safe, personalized diabetes management decisions” and may be “more clinically relevant for making management decisions for an individual.”
That said, a 2024 perspective published in Diabetes Care argued that GMI “has outlived its usefulness” as an A1c estimate, precisely because so many people see a significant gap between the two. The authors pointed out that presenting GMI in the same percentage format as A1c creates the impression they’re interchangeable, which could lead some people to skip lab A1c testing altogether. That would be a mistake, since A1c has decades of research linking it to long-term complications like kidney disease, nerve damage, and cardiovascular problems. CGM-based metrics like GMI don’t yet have that same body of evidence behind them.
How to Use GMI Wisely
Think of GMI as a useful directional signal, not a replacement for lab work. If your GMI has been trending downward over the past few weeks, your glucose management is likely improving. If it’s creeping up, something may have shifted in your routine worth examining.
If your GMI and lab A1c consistently differ by more than half a percentage point, that’s normal for a sizable chunk of the population. It doesn’t mean your sensor is broken or your lab result is wrong. It means your body’s hemoglobin glycation rate is slightly different from the population average the GMI formula was built on. Once you know your personal offset, you can mentally adjust when reading your GMI.
Other reports in the Libre 3 app, particularly Time in Ranges and Daily Patterns, often provide more actionable information than GMI alone. Time in Range tells you what percentage of the day your glucose stays between 70 and 180 mg/dL, which many clinicians now consider just as important as A1c for evaluating glucose control. Used together, these metrics paint a more complete picture than any single number.