GMI explained: estimating A1c from CGM data
GMI (glucose management indicator) is an estimate of A1c worked out from the average glucose on a continuous glucose monitor. The formula is GMI (%) = 3.31 + 0.02392 × average glucose in mg/dL. For an average of 154 mg/dL it gives about 7.0%. It can differ from your laboratory A1c, which is normal.
What is GMI?
The glucose management indicator was introduced in 2018 to replace the older term "estimated A1c" for sensor data. It turns the average glucose from a continuous glucose monitor into a percentage that looks like an A1c.
What is the GMI formula?
- Average in mg/dL, result in %: GMI (%) = 3.31 + 0.02392 × average glucose
- Average in mmol/L, result in mmol/mol: GMI (mmol/mol) = 12.71 + 4.70587 × average glucose. To get a percentage, use the A1c converter.
| Average (mg/dL) | Average (mmol/L) | GMI (%) |
|---|---|---|
| 100 | 5.5 | 5.7 |
| 120 | 6.7 | 6.2 |
| 140 | 7.8 | 6.7 |
| 154 | 8.5 | 7.0 |
| 160 | 8.9 | 7.1 |
| 180 | 10.0 | 7.6 |
| 200 | 11.1 | 8.1 |
| 220 | 12.2 | 8.6 |
Why can GMI differ from a lab A1c?
- The lab A1c reflects about three months. A GMI reflects only the days your sensor was worn.
- People differ in how much glucose attaches to haemoglobin at the same average glucose.
- Some health conditions affect A1c accuracy.
A gap between the two numbers does not mean either one is wrong. Your care team can tell you what it means for you.
Where do I find my GMI?
Most sensor apps and reports show it next to your average glucose and time in range. The average glucose calculator on this site also shows it for the readings you enter, with the same 14-day caveat.
Information, not medical advice. Talk to your care team about your own targets and treatment. Never use a converter or a diary entry to work out an insulin or medicine dose.
Sources
- Bergenstal et al., Diabetes Care, 2018: Glucose Management Indicator (GMI)
- Nathan et al., Diabetes Care, 2008: Translating the A1C assay into estimated average glucose values
- NIDDK: Diabetes tests and diagnosis
Frequently asked questions
How do I calculate GMI in mmol/L?
Multiply your average glucose in mmol/L by 4.70587 and add 12.71. This gives GMI in mmol/mol. For example, an average of 8.6 mmol/L gives about 53 mmol/mol, which is about 7.0%.
Why is my GMI different from my lab A1c?
GMI uses the glucose recorded by your sensor over the days measured. A lab A1c reflects about three months, and people differ in how glucose attaches to haemoglobin, so the two numbers often differ. Your care team can explain what it means for you.
How many days of data does GMI need?
It is meant for continuous glucose monitor data covering at least 14 days.
Keep reading
A1c vs daily readings
How A1c differs from daily blood sugar readings, what estimated average glucose (eAG) means, and why the two numbers do not always match.
Blood sugar basicsTime in range
Time in range is the share of the day your glucose stays between 70 and 180 mg/dL. How it is calculated and what the international consensus says.
Free toolA1c converter
Convert A1c between percent (NGSP) and mmol/mol (IFCC) and see the estimated average glucose. Free, private, with formulas and sources.