Understand

mg/dL vs mmol/L

The same drop of blood, the same glucose — written down as two very different numbers. The US measures blood sugar in mg/dl, most of the world in mmol/L, and the conversion is a single division: mg/dl ÷ 18 = mmol/L. This page shows who uses which unit, the landmark values in both, and how to avoid the classic mix-up.

Who uses which unit

Milligrams per deciliter (mg/dl) is a weight-per-volume unit; millimoles per liter (mmol/L) counts molecules instead. Both describe exactly the same measurement.

• mg/dl: the United States, plus parts of continental Europe — Germany is famously split, with the west of the country mostly using mg/dl and the east mmol/L. Also common in Japan, India and much of Latin America.
• mmol/L: the UK, Canada, Australia, New Zealand, China and most of the rest of the world. It is also the unit used in most international research papers.

Meters sold in one country are usually locked to that country's unit, but manuals, apps and websites cross borders freely — which is where confusion starts.

The conversion: divide or multiply by 18

One mmol of glucose weighs about 180 mg, and a deciliter is a tenth of a liter — so the two units differ by a factor of 18 (18.016 exactly, but 18 is accurate enough for anything outside a laboratory).

• mg/dl → mmol/L: divide by 18. 126 ÷ 18 ≈ 7.0
• mmol/L → mg/dl: multiply by 18. 5.5 × 18 = 99

A useful mental shortcut: mmol/L values are roughly the mg/dl value with the last digit dropped, halved — 180 mg/dl → 18 → about 10 mmol/L. Close enough to sanity-check a number.

The landmark values in both units

These six numbers carry almost all the clinical meaning (ADA classification):

The full context — why the same number classifies differently fasting versus after eating — is covered in normal blood sugar levels.

How to tell which unit a number is in

Usually the number itself gives it away:

• Below about 35 → mmol/L. Real mg/dl readings this low are barely compatible with consciousness; real mmol/L readings above 33 (≈600 mg/dl) are a medical emergency and rare. A "7.2" or a "15" is almost certainly mmol/L.
• A decimal point → almost always mmol/L. mmol/L values are quoted to one decimal; mg/dl readings are whole numbers, typically 40–400 on a home meter.
• Between roughly 35 and 40 is the only genuinely ambiguous zone, and numbers there are alarming in either unit — treat them as a reason to re-test and act, not to debate units.

The classic mix-up

The mix-up runs the other way too: a European reader seeing an American forum post about "blood sugar 100" may briefly wonder about an extreme emergency (100 mmol/L does not occur in a living person) when the writer meant a perfectly normal fasting reading. Whenever a number seems impossible, check the unit before checking anything else. Genuinely dangerous readings — above 250 mg/dl (13.9 mmol/L)or below 54 mg/dl (3.0 mmol/L) — are covered on their own pages.

Working with both units in practice

• The checker on this site accepts both units and auto-detects them. Enter 5.5 or 99 — low values with decimals are read as mmol/L, and every result page shows both units side by side.
• Know your meter's unit and whether it can be switched. Some meters can be toggled accidentally; if your readings suddenly look 18 times too small, that is what happened, not your pancreas.
• Log readings in one unit consistently — the on-device journal here keeps entries comparable, which is what makes patterns visible.
• Talking to a doctor abroad? Give the number with its unit ("126 milligrams per deciliter"), or convert first. Divide by 18 going out of the US, multiply by 18 coming in.
• Not sure your technique is giving you honest numbers in the first place? See how to test blood sugar at home.