Glucose management indicator: GMI (%) = 3.31 + 0.02392 × average CGM glucose in mg/dL.
GMI is written in A1c units, so it is tempting to read it against the American Diabetes Association A1c bands of 5.7% to 6.4% for the prediabetes range and 6.5% or above for the diabetes threshold. Those bands are defined on a lab A1c, not on GMI, so a GMI in either range is a reason to compare with a lab test rather than a label in itself.
Working the formula backward, a GMI of 5.7% matches an average of about 100 mg/dL, 6.5% about 133 mg/dL and 7% about 154 mg/dL. The International Consensus on Time in Range adds a second lens. For most adults with type 1 or type 2 diabetes it suggests more than 70% of readings between 70 and 180 mg/dL, fewer than 4% below 70 and fewer than 25% above 180.
GMI tells you about the average and nothing else. Two people with the same GMI can have very different days, one steady and one swinging high and low, which is why the consensus report places GMI next to time in range and glucose variability rather than above them.
The input is the monitor average, so the amount and quality of data matter first. The consensus recommends 14 days of wear with at least 70% of possible readings captured. Gaps during sleep, travel or exercise can tilt the average toward whatever hours happened to be recorded.
Short term changes move GMI faster than they move a lab A1c. The GMI authors note that a new exercise routine, an illness, or the first few weeks of an effective glucose lowering medicine can pull GMI away from a recent lab result for a while, because the lab value still carries the older weeks.
On the lab side, A1c depends on how long red blood cells live and how readily glucose attaches to hemoglobin, and that differs from person to person. Hemoglobin variants and hemolytic anemia are named as reasons a lab A1c can sit well above or below GMI even when the monitor is accurate.
GMI refreshes with every new 14 day window, so there is no single retest date. The most useful comparison is a lab A1c drawn at the end of a period with good monitor coverage and no big recent change in routine, which lets you see how your own two numbers line up.
Timing matters for that comparison. A lab A1c reflects the past few months, while GMI covers only the most recent two weeks. When glucose has been trending up or down, the two will drift apart for reasons that have nothing to do with red cells, so a fair comparison uses a stretch when the monitor average has been fairly steady for a while.
The GMI authors report that the gap between GMI and lab A1c tends to stay fairly stable for each person over time, so once you know your usual gap you can read later GMIs with it in mind. A gap that suddenly changes, or frequent readings below 54 mg/dL, which the consensus asks to keep under 1% of the time for most adults with diabetes, is worth raising with a clinician promptly.
In a comparison of 528 people with diabetes, GMI and lab A1c matched to within 0.1 points only 19% of the time. They differed by 0.3 points or more in 51% of cases, by 0.5 points or more in 28%, and by a full point or more in 3%. A gap of half a point is large enough to move someone across a guideline band.
The data came from people with diabetes wearing modern sensors, so GMI is less studied in people without diabetes, in pregnancy and in children. The formula is designed for continuous monitor averages, not for fingerstick logs, and it assumes the sensor itself reads accurately across the whole range. It also assumes the 14 days are typical of the months that a lab A1c covers.
GMI also cannot say when highs and lows happen. A steady 140 mg/dL and a day split between long highs and repeated lows can produce the same average and the same GMI, which is why the consensus report reads it beside time below range and variability.
Say a monitor report shows an average glucose of 140 mg/dL across 14 days with good wear time. The calculator runs GMI as 3.31 plus 0.02392 times the average. That is 0.02392 times 140, which gives 3.3488, and 3.31 plus 3.3488 gives 6.6588, shown rounded as 6.7%.
In plain words, a lab A1c drawn now might land somewhere near 6.7%, though for roughly half of people it would differ by 0.3 points or more. Read against the A1c bands, 6.7% sits just above the 6.5% threshold. That makes the next lab A1c, the time in range figure and a conversation with a clinician the useful next steps, not the GMI alone.
GMI and estimated average glucose are mirror images built from different data. The older ADAG equation turns an A1c into an average glucose, using fingersticks and earlier sensors from 507 adults. Turned around, it puts an average of 140 mg/dL at an A1c of about 6.5%, slightly lower than the 6.7% GMI gives for the same average.
A lab A1c is still the test guidelines use for thresholds and long term goals. GMI adds what A1c cannot show, a current picture that updates every two weeks, and time in range adds how much of the day is spent in target. Read together, the three give a fuller view than any one alone, and disagreements between them are a prompt for questions rather than a verdict.
The Metabolic and insulin panel measures the values this calculator uses. Related: hemoglobin a1c, eag, fasting glucose.
A1c is measured in a blood sample. GMI is calculated from CGM glucose readings. They often agree but are not the same test.
The formula was built from CGM data, so it is designed for CGM averages over 14 days or more.
In the published GMI table, an average of 150 mg/dL corresponds to a GMI of 6.9% and 175 mg/dL to 7.5%.