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GMI to A1C Calculator (CGM)
If you use a continuous glucose monitor (Libre, Dexcom), your report shows a Glucose Management Indicator (GMI) — an estimate of A1C computed from your CGM mean glucose with the Bergenstal 2018 equation, GMI% = 3.31 + 0.02392 × mean glucose (mg/dL). The crucial point most tools miss: GMI is not the same as a lab A1C. It estimates A1C, but roughly 1 in 5 people differ by half a percentage point or more, because a lab A1C reflects hemoglobin glycation, not just your CGM average. This calculator takes your CGM mean glucose, shows the GMI, and keeps that distinction front and centre so you never mistake a CGM number for your official A1C. Enter your mean glucose in mg/dL or mmol/L. Free, private, browser-only.
GMI% = 3.31 + 0.02392 × CGM mean glucose (mg/dL) — Bergenstal 2018
- Worked example: a CGM mean of 150 mg/dL → GMI ≈ 6.9% (not necessarily your lab A1C)
- eAG ≠ GMI: a lab A1C of 7% implies eAG ≈ 154 mg/dL, above the 150 mean here
Frequently asked questions
Is GMI the same as A1C?
No. GMI (Glucose Management Indicator) is a CGM-derived estimate of A1C using GMI% = 3.31 + 0.02392 × mean glucose (mg/dL). It commonly differs from a lab A1C — about 1 in 5 people by 0.5 percentage points or more — because A1C measures glycated hemoglobin directly. Use GMI to track CGM trends, but rely on a lab A1C for diagnosis and clinical decisions.
Why is my GMI different from my lab A1C?
They come from different things. GMI reflects your average CGM glucose over the reporting window (14+ days preferred). A lab A1C reflects hemoglobin glycation over ~3 months and is influenced by red-cell lifespan, so anemia, hemoglobinopathies, pregnancy or kidney disease can shift it away from the CGM estimate. A gap of a few tenths of a percent is normal.