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Blood Glucose Units: mg/dL and mmol/L Explained

Two different unit systems are used worldwide to report blood glucose, and knowing the conversion factor helps make sense of a number from either one. This is general information, not medical advice.

mg/dL measures a mass concentration

Milligrams per deciliter (mg/dL) states how many milligrams of glucose are dissolved in one deciliter of blood. This is the standard unit used in the United States and a small number of other countries.

mmol/L measures a molar concentration

Millimoles per liter (mmol/L) is the SI-based unit used across most of the world, including the UK, Canada, Australia, and much of Europe and Asia. It counts glucose molecules rather than their mass.

The conversion factor is glucose's molar mass, about 18

To convert mg/dL to mmol/L, divide by 18.016 (commonly rounded to 18). To go the other direction, multiply mmol/L by 18. For example, 100 mg/dL divided by 18 is about 5.6 mmol/L.

Reference ranges describe the same physiology in both units

A commonly cited fasting reference range of roughly 70-99 mg/dL corresponds to roughly 3.9-5.5 mmol/L β€” the same underlying glucose levels, just expressed on two different numeric scales.

A1C is a separate measurement, not a glucose unit conversion

Hemoglobin A1C is reported as a percentage and reflects average blood glucose over roughly the past 2-3 months, unlike mg/dL or mmol/L readings, which capture glucose at a single point in time β€” the two are related but not interchangeable by a simple formula.

Why the two systems never merged

mg/dL took hold early in US clinical laboratory practice and became entrenched in American medical recordkeeping and patient-facing devices, while the SI-based mmol/L was adopted more broadly as international standards bodies pushed for molar-based reporting across chemistry and medicine. Because both systems are deeply embedded in their respective countries' healthcare infrastructure, switching either one over would require retraining, re-labeling devices, and rewriting reference materials at a massive scale.

Reading a device or app set to the "wrong" unit

Travelers, people using an imported glucose meter, or anyone reading international medical literature will encounter the other unit system at some point. Because the conversion is a simple fixed multiplication or division by about 18, it is one of the more straightforward unit conversions to sanity-check mentally, unlike many other unit pairs that involve more complex formulas.

Frequently Asked Questions

Is 100 mg/dL the same as 5.6 mmol/L?

Yes, approximately β€” 100 divided by 18.016 is about 5.55 mmol/L, commonly rounded to 5.6. Both numbers describe the same blood glucose concentration, just in different units.

Which countries use mmol/L instead of mg/dL?

Most of the world, including the UK, Canada, Australia, and most of Europe and Asia, uses mmol/L. The US is one of the more notable countries that primarily uses mg/dL instead.