Glucose Management Indicator (GMI) Calculator
Estimate the glucose management indicator (GMI) — an approximation of HbA1c derived from the mean glucose measured by a continuous glucose monitor (CGM). GMI replaces the older term “estimated A1c” and is a standard part of the ambulatory glucose profile. Enter the average sensor glucose over at least 14 days.
About the GMI
The glucose management indicator estimates HbA1c from CGM mean glucose: GMI (%) = 3.31 + 0.02392 × mean glucose (mg/dL). Source: Bergenstal RM et al., Diabetes Care 2018;41(11):2275–2280.
Frequently asked questions
What is the GMI formula?
GMI (%) = 3.31 + 0.02392 × mean glucose in mg/dL. In mmol/L the equivalent is GMI (%) = 3.31 + 0.02392 × (mean glucose × 18). This calculator also reports GMI in mmol/mol (IFCC units). For example, a mean glucose of 154 mg/dL gives a GMI of about 7.0%.
How is GMI different from HbA1c?
GMI is an estimate of what your laboratory HbA1c is likely to be, calculated from CGM data over a period of days; HbA1c is measured directly from a blood sample and reflects roughly three months. The two often differ by a few tenths of a percent because red-cell turnover and other factors affect measured A1c but not GMI. Because GMI depends only on the mean sensor glucose, it moves as soon as your average glucose moves, whereas a lab A1c lags weeks behind a change in control. That makes GMI the more responsive number to review between appointments and HbA1c the one used for diagnosis and formal reporting.
How much CGM data do I need?
GMI is most reliable from at least 14 days of continuous glucose data with good sensor wear (about 70% or more of readings captured). Shorter or patchy periods give a less representative mean glucose and therefore a less trustworthy GMI. Fourteen days is the window used for the standard ambulatory glucose profile, so the same period also underpins the other core CGM metrics such as time in range and time below range. If your sensor wear falls well short of that, treat the GMI shown here as indicative and repeat it once you have a fuller record.
What is a good GMI?
GMI is interpreted like A1c. A common general target for many non-pregnant adults is below 7.0%, but goals are individualised — some people aim lower and others higher depending on age, risks and hypoglycaemia. Discuss your own target with your care team.
Does GMI replace a lab A1c?
GMI does not replace a measured HbA1c. It is a helpful, timely estimate between visits, but it is not a laboratory result and does not replace clinical judgement. When GMI and lab A1c disagree markedly, the difference itself is useful information. This calculator is an educational reference, not medical advice.
This is a reference tool, not a diagnosis or medical advice. Results depend on the laboratory, assay and clinical context, and reference intervals vary between labs and by age and sex — always read your result against the range printed on your own report and discuss it with a qualified healthcare professional.
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