mg/dL vs. mmol/L: Why Blood Sugar Has Two Units

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If you've ever compared a glucose meter reading with someone from another country, you may have noticed the numbers look completely different — even for the same blood sugar level. That's because the world uses two different measurement systems for blood glucose.

Two units, one measurement

In the United States, blood glucose is typically reported in milligrams per deciliter (mg/dL) — a measure of weight of glucose per volume of blood. Most of the rest of the world, including the UK, Canada, and much of Europe and Asia, uses millimoles per liter (mmol/L) — a measure of the molar concentration of glucose.

Both describe the same underlying quantity of sugar in the blood; they're just different scales, similar to Fahrenheit and Celsius for temperature.

The conversion formula

To convert mg/dL to mmol/L, divide by 18. To go the other direction, multiply mmol/L by 18. For example, a reading of 126 mg/dL is about 7.0 mmol/L. Our glucose unit converter does this instantly in either direction.

Why it matters

If you travel internationally, use an app or device built for a different region, or read research from outside the US, you'll likely encounter mmol/L. Misreading a mmol/L number as if it were mg/dL (or vice versa) can make a normal reading look alarmingly high or low, so it's worth double-checking which unit a device or article is using.

How this relates to A1C

A1C is reported as a percentage rather than in either glucose unit, but it can be translated into an estimated average glucose in mg/dL or mmol/L using a published formula. See what is an A1C test and try the A1C to eAG converter to see how the two connect.

A quick mental check

If you're ever unsure which unit you're looking at, a rough gut-check helps: normal fasting glucose is under 100 in mg/dL, but under 5.6 in mmol/L. If a "normal" fasting number looks like it's in the single digits or low teens, you're almost certainly looking at mmol/L, not mg/dL — the two scales don't overlap in a way that's easy to confuse once you know the rough ranges.

Why two systems exist

The difference traces back to broader measurement conventions: mg/dL reflects a mass-based approach common in US clinical medicine, while mmol/L reflects the molar concentration approach used in the International System of Units (SI), adopted more broadly by medical systems outside the US. Neither is more "correct" — they're just different conventions that never fully converged internationally.

Converting on the go

Beyond a dedicated converter, a rough mental shortcut can help in a pinch: dividing a mg/dL number by 18 gets you close to mmol/L, and multiplying a mmol/L number by 18 gets you back to mg/dL. It's not exact enough for clinical decisions, but it's a useful sanity check when reading a foreign nutrition label or a friend's meter while traveling. For anything that matters, our converter gives you the precise figure instantly.

Sources

American Diabetes Association: Blood Glucose Testing