What is my GMI?

Medical disclaimer

This calculator is for information and education only. It is not medical advice, and its result is not a diagnosis. Do not use it to start, stop, or change a treatment without a clinician who knows your health history. No licensed clinician has reviewed this page. This site's editors checked the formulas and worked examples against published sources. If you think you have a medical emergency, call your local emergency number.

When this does not apply. The formula comes from 528 people with type 1 or type 2 diabetes in four studies that used Dexcom CGM sensors (Bergenstal 2018); the authors say it may differ a little for other sensors. It needs at least 10, and preferably 14 or more, days of CGM data. The GMI is an estimate, not a lab A1C. It can differ from a lab A1C after a recent change in glucose levels, and in conditions that change the life span of red blood cells. The paper's Table 1 runs from 100 to 350 mg/dL and from 5 to 18 mmol/L; this page accepts 90 to 350 mg/dL or 5 to 19.4 mmol/L and gives no answer outside that range.

How it works

With the mean glucose G from the CGM report:

• GMI (%) = 3.31 + 0.02392 × G, with G in mg/dL;
• GMI (mmol/mol) = 12.71 + 4.70587 × G, with G in mmol/L.

Both formulas are from Bergenstal 2018 (Figure 1 and Table 1). The page always shows both:

• If you type mg/dL, the percent uses the typed value, and the mmol/mol uses G ÷ 18.
• If you type mmol/L, the mmol/mol uses the typed value, and the percent uses G × 18.

1 mmol/L of glucose is taken as exactly 18 mg/dL (180.16 g/mol ÷ 10 = 18.016, rounded to 18, as this site does for glucose). The page also shows the glucose in the other unit: mg/dL ÷ 18 to 1 decimal, or mmol/L × 18 to a whole number.

Rounding. The page works with the exact typed decimal and rounds each result once, halves up, at the end: each GMI to 1 decimal. The rounded value is the result (so 112.5 mg/dL, which is exactly 6.25 mmol/L, shows as 6.3 mmol/L).

Allowed input. 90 to 350 mg/dL, or 5 to 19.4 mmol/L (19.4 mmol/L is 349.2 mg/dL; the mmol/L bound is kept to one decimal, so it stops 0.8 mg/dL short of 350). A value outside this range gives no answer.

Assumptions

• The mean glucose comes from at least 10, and preferably 14 or more, days of CGM readings.
• The GMI is an estimate for education. It is not a lab A1C and not medical advice.

Worked examples by hand

Page default, 150 mg/dL. GMI = 3.31 + 0.02392 × 150 = 3.31 + 3.588 = 6.898, shown as 6.9% (Table 1: 6.9). In mmol/L, 150 ÷ 18 = 8.333, so GMI = 12.71 + 4.70587 × 8.333 = 51.926, shown as 51.9 mmol/mol.

100 mg/dL. 3.31 + 2.392 = 5.702, shown as 5.7% (Table 1: 5.7). 100 ÷ 18 = 5.556 mmol/L; 12.71 + 26.144 = 38.9 mmol/mol.

350 mg/dL. 3.31 + 8.372 = 11.682, shown as 11.7% (Table 1: 11.7). 350 ÷ 18 = 19.444 mmol/L; 12.71 + 91.503 = 104.2 mmol/mol.

5 mmol/L. GMI = 12.71 + 4.70587 × 5 = 36.239, shown as 36.2 mmol/mol (Table 1: 36.2). 5 × 18 = 90 mg/dL; 3.31 + 2.1528 = 5.4628, shown as 5.5%.

10 mmol/L. 12.71 + 47.0587 = 59.769, shown as 59.8 mmol/mol (Table 1: 59.8). 180 mg/dL gives 3.31 + 4.3056 = 7.6156, shown as 7.6%.

18 mmol/L. 12.71 + 84.706 = 97.416, shown as 97.4 mmol/mol (Table 1: 97.4). 324 mg/dL gives 3.31 + 7.750 = 11.060, shown as 11.1%.

112.5 mg/dL. 3.31 + 2.691 = 6.001, shown as 6.0%. 112.5 ÷ 18 = 6.25 mmol/L exactly, shown as 6.3; 12.71 + 4.70587 × 6.25 = 42.122, shown as 42.1 mmol/mol.