estimatea1c.com › Guides › GMI vs A1C
If you wear a continuous glucose monitor, you've probably noticed two numbers that are supposed to mean the same thing but don't quite match: the GMI your app shows, and the A1C from your last blood draw. A gap of a few tenths of a percent is normal — and understanding why tells you which number to actually act on.
| GMI | A1C | |
|---|---|---|
| Comes from | Your CGM's average glucose | A blood test |
| Time window | Last 14+ days | Past ~2–3 months |
| Measures | Average glucose, directly | Glucose stuck to hemoglobin |
| Best for | Tracking recent change | The clinical standard of record |
They disagree because they measure glucose exposure in fundamentally different ways. One reads your sugar directly; the other reads a downstream effect of it on your blood cells.
GMI — Glucose Management Indicator — converts your CGM's mean glucose into an A1C-like percentage, using the formula from Bergenstal and colleagues (2018). It needs at least 14 days of CGM data (ideally with the sensor active 70%+ of the time) to be reliable.
Enter your CGM's mean glucose for the period.
GMI (%) = 3.31 + 0.02392 × mean glucose (mg/dL)
A1C (HbA1c) measures the percentage of your hemoglobin that has glucose chemically bound to it. Because red blood cells live about three months, A1C is a slow, integrated picture of your glucose — weighted toward the most recent weeks. It's what diagnosis and treatment guidelines are built on. See our A1C ⇄ average glucose calculator to translate any A1C into everyday units.
GMI is pure arithmetic on your glucose average. A1C depends on biology that varies from person to person: how long your red blood cells live, how quickly glucose attaches to them, and conditions like anemia, kidney disease, pregnancy, or certain hemoglobin variants. Two people with identical average glucose can have measurably different lab A1Cs. Clinicians call this the "glycation gap," and it commonly produces a 0.3–0.5% difference — sometimes more — in either direction.
Use both, for different jobs. GMI updates within days, so it's the better steering wheel when you're changing habits and want to see whether your average is moving. A1C is the official scorecard your care team treats to. If your GMI runs consistently lower than your lab A1C, don't assume the lab is wrong — track the gap and bring it up at your next visit.
Flag it with your clinician if GMI and A1C disagree by more than ~0.5% repeatedly, if either jumps unexpectedly, or if you have a condition affecting red blood cells. The gap can occasionally point to something worth investigating, like anemia.
SteadyRange logs your glucose and weight, shows your time in range and estimated A1C as they trend, and exports a clean PDF for your next appointment. Private by default.
Try SteadyRange →No. Both come from average glucose, but eAG is derived from your A1C (the ADAG formula) and shown in mg/dL or mmol/L, while GMI is derived from your CGM's mean glucose and shown as a percentage. They're close cousins, not identical.
Your red-blood-cell biology glycates a bit more than average, so your lab A1C lands above what your glucose average alone predicts. It's common and usually not a problem — just track the gap.
Neither is "wrong." GMI is a precise estimate of your recent average glucose; A1C is the measured clinical standard. For day-to-day steering, GMI is more responsive; for diagnosis and treatment targets, A1C rules.
At least 14 days, with the sensor collecting data most of the time (about 70%+). Fewer days makes GMI noisy.
Medical disclaimer. General education, not medical advice. GMI and eAG are population-based estimates and may not match your measured lab A1C. Do not change any treatment based on this page. Discuss your numbers with a qualified healthcare provider.