GMI (Glucose Management Indicator) is an estimate of your A1C calculated from your CGM readings, while A1C is a blood test that measures your average glucose over the past 2 to 3 months. They answer the same question in different ways, so they often land close together but rarely match exactly. A gap of a few tenths of a percent (or even a full point) between GMI and A1C is normal and does not mean either number is wrong.
The short version: GMI comes from your CGM sensor data over a set number of days, and A1C comes from a lab blood draw. Use GMI to spot trends between lab visits, and use A1C as the standard your care team tracks over time.
What is A1C?
A1C (also written HbA1c) is a blood test that shows the percentage of your hemoglobin, a protein in red blood cells, that has glucose attached to it. Because red blood cells live about 3 months, A1C reflects your average glucose over roughly the past 2 to 3 months.
The American Diabetes Association uses A1C as a core measure of long-term glucose control. It is drawn in a lab or clinic, usually a few times a year, and it does not require you to fast.
Why A1C can mislead
A1C assumes your red blood cells behave in an average way. Several things can throw it off:
- Anemia, recent blood loss, or a recent transfusion
- Pregnancy
- Certain hemoglobin variants, more common in some ethnic groups
- Kidney disease or conditions that change red blood cell lifespan
A1C also hides the story behind the average. Two people can share the same A1C while one has steady, in-range days and the other swings between highs and lows.
What is GMI?
GMI, the Glucose Management Indicator, is a number calculated from the average glucose your CGM records over a period of time, usually 14 days. It was introduced in 2018 to give CGM users a familiar A1C-style percentage without waiting for a lab draw.
GMI is displayed as a percentage in the United States (for example 6.8%) and can also appear in mmol/mol. Because it comes straight from sensor data, you can see it update on your CGM report at any time.
The formula behind GMI
GMI is derived from your average sensor glucose. A higher average glucose produces a higher GMI. The important point is that GMI is only as good as the data behind it: you generally need at least 14 days of CGM wear, with the sensor active most of the time, before GMI reflects your real pattern.
Why do GMI and A1C differ?
GMI and A1C measure related but not identical things, so a difference is expected. Common reasons include:
- Different time windows. GMI usually covers the last 14 days. A1C reflects 2 to 3 months. If your glucose recently improved or worsened, the two will disagree.
- Individual biology. Some people’s red blood cells hold onto glucose more or less than average. This gives them an A1C that is consistently higher or lower than their GMI.
- Incomplete CGM data. If your sensor missed readings, warmed up often, or you wore it less than about 70% of the time, GMI may not represent your full picture.
- Lab and sensor accuracy. Both tools have a margin of error, and small differences add up.
Research behind GMI found that for many people the two numbers land within about 0.3%, but a meaningful share of people see gaps larger than that. If your GMI and A1C differ by more than half a point again and again, tell your care team. That consistent gap is itself useful information for personalizing your goals.
Which number should you trust?
Neither one is the single truth. They work best together.
- Use A1C as the standard reference your healthcare provider tracks over years and uses to guide care.
- Use GMI to check your direction between visits, so you are not surprised at your next appointment.
Think of A1C as a photo taken every few months and GMI as a live preview you can glance at any day. If GMI is trending down over several weeks, your next A1C will likely follow.
GMI, A1C, and Time in Range together
GMI and A1C both compress months of data into one number, which means they still hide your day-to-day swings. That is where Time in Range adds value. Time in Range shows the percentage of the day your glucose stays between 70 and 180 mg/dL (3.9 to 10.0 mmol/L), so you can see stability, not just an average.
The 2019 International Consensus on Time in Range suggests many adults aim for at least 70% of the day in range, though your personal target should come from your care team. Reading GMI, A1C, and Time in Range side by side gives a fuller picture than any single number. To learn how the range metric works, see our guide to Time in Range explained.
How to see your GMI
Most CGM reports show GMI automatically once you have enough sensor data. You will usually find it on a summary page alongside your average glucose and Time in Range. Manufacturer apps and clinic software both display it.
If your GMI looks off, first check how many days of data it is based on and whether your sensor had gaps. A CGM number is only trustworthy when the sensor is reading well, which is also why your CGM reading can differ from a finger stick in the moment.
Frequently asked questions
Is GMI the same as estimated A1C (eA1C)?
GMI replaced the older term “estimated A1C.” Experts moved to “GMI” because it comes purely from CGM glucose data and should not be treated as an exact prediction of your lab A1C. The names describe similar ideas, but GMI is the current, preferred term.
How many days of CGM data do I need for an accurate GMI?
Aim for at least 14 days of wear with the sensor active roughly 70% of the time or more. With less data, GMI can swing and may not reflect your true average.
My GMI is lower than my A1C. Is that bad?
Not necessarily. Some people naturally run an A1C higher than their glucose average would predict, and the reverse is also true. A steady gap is worth discussing with your provider so they can set goals that fit your body, but it is not automatically a problem.
Can GMI replace an A1C lab test?
No. GMI is a helpful companion, but your care team still relies on lab A1C as the validated standard. Keep getting the lab test as recommended.
Sugar Sense pulls your real-time CGM readings and helps you keep an eye on Time in Range and daily trends between clinic visits, so your GMI never comes as a surprise. If you want your glucose data and patterns in one place on your iPhone and Apple Watch, see how Sugar Sense works.
This article is educational and general in nature. Sugar Sense is a companion app, not a medical device, and it does not replace your CGM, its alarms, or a lab A1C test. Always confirm your numbers with your CGM device and make treatment and goal decisions with your healthcare provider. See our medical disclaimer.
