Glucose Converter

Q: Why do some countries use mg/dL and others mmol/L for blood glucose?

The United States, Canada (historically), and several other countries adopted milligrams per deciliter (mg/dL) from the older gravimetric measurement tradition used in early 20th-century clinical chemistry. Most of the world — including the UK, Australia, India, Europe, and New Zealand — switched to the SI (Système International) unit millimoles per litre (mmol/L) when international scientific bodies standardized laboratory reporting in the 1970s and 1980s. Both measure exactly the same thing; only the scale differs.

Q: Which countries use mg/dL vs mmol/L?

mg/dL is used primarily in the United States and in some older clinical systems in Canada and parts of Latin America. mmol/L is the standard in the United Kingdom, Australia, New Zealand, most of Europe, India, China, Japan, South Africa, and the majority of countries that follow SI laboratory conventions. If your glucometer or CGM app was purchased in the US, it almost certainly shows mg/dL by default; international devices default to mmol/L.

Q: Are CGM apps like Dexcom and Libre available in both units?

Yes. Dexcom G6/G7, Abbott FreeStyle Libre 2/3, and Medtronic Guardian CGM apps all support both mg/dL and mmol/L display. The unit can be changed in the app settings — look under "Display Units" or "Glucose Units." Note that if you share CGM data with a clinician in a different country, you may need to convert values manually for interpretation. Some third-party apps such as Spike and Nightscout also let you switch units freely.

Q: What is a dangerous blood glucose level?

According to the ADA, severe hypoglycemia (dangerously low) is defined as below 54 mg/dL (3.0 mmol/L) — this level can impair cognition and requires immediate treatment. Mild-to-moderate hypoglycemia begins below 70 mg/dL (3.9 mmol/L). On the high end, a fasting glucose of 126 mg/dL (7.0 mmol/L) or above on two separate occasions meets the ADA diagnostic threshold for diabetes. A random glucose above 200 mg/dL (11.1 mmol/L) with symptoms (polyuria, polydipsia) is also diagnostic. Hyperglycemic crisis (DKA) typically involves levels above 250 mg/dL (13.9 mmol/L). Always consult your clinician — context (fasting vs. post-meal, timing, symptoms) matters enormously.

Q: How do I convert mg/dL to mmol/L without an app?

Divide the mg/dL value by 18. This is a close approximation of the exact factor 18.0182 (derived from glucose's molar mass of 180.156 g/mol). For example, 126 mg/dL ÷ 18 = 7.0 mmol/L. To go the other direction — mmol/L to mg/dL — multiply by 18. So 5.5 mmol/L × 18 = 99 mg/dL. The rounding error using 18 instead of 18.0182 is less than 0.1% and is clinically insignificant.

Context	Category	mg/dL	mmol/L
Any time	Severe Hypo	<54	<3.0
Fasting	Normal	70–99	3.9–5.5
Fasting	Prediabetes	100–125	5.6–6.9
Fasting	Diabetes	≥126	≥7.0
2-h OGTT	Normal	<140	<7.8
2-h OGTT	Prediabetes	140–199	7.8–11.0
2-h OGTT	Diabetes	≥200	≥11.1

Why do two units for blood glucose exist?

Blood glucose is the same molecule everywhere in the world. The difference between mg/dL and mmol/L is purely one of measurement convention — a legacy of two competing laboratory traditions that were never unified before glucometers became consumer devices.

The United States adopted milligrams per deciliter (mg/dL) from the gravimetric chemistry tradition dominant in North American clinical labs through the mid-20th century. The rest of the world — Europe, the UK, Australia, India, Japan, and most countries following international scientific standards — switched to millimoles per litre (mmol/L) when the Système International d'Unités (SI) was standardized for laboratory medicine in the 1970s and 1980s. The SI unit expresses concentration in terms of the number of molecules (moles) rather than mass, which is conceptually cleaner for chemistry. But the US healthcare system, with its installed base of legacy lab infrastructure and clinical guidelines written in mg/dL, never made the switch.

The practical consequence: a US traveler using a domestic glucometer abroad, a patient reading a European clinical study, or an immigrant comparing their home-country diabetes records with a new clinician — all routinely encounter values in an unfamiliar unit.

The 18.0182 conversion factor — where it comes from

The exact conversion factor between mg/dL and mmol/L is 18.0182, derived directly from glucose's molar mass.

Glucose (C₆H₁₂O₆) has a molecular weight of 180.156 g/mol. One millimole of glucose weighs 180.156 milligrams. A deciliter is one-tenth of a litre, so 1 mmol/L = 0.1 mmol/dL = 18.0156 mg/dL. The factor 18.0182 accounts for the precise atomic weights used in modern IUPAC tables. The two formulas:

mg/dL → mmol/L: divide by 18.0182
mmol/L → mg/dL: multiply by 18.0182

For mental math, rounding to 18 introduces an error of only 0.1% — clinically irrelevant for everyday interpretation.

ADA classification thresholds

The American Diabetes Association's Standards of Medical Care in Diabetes — 2024 defines fasting plasma glucose thresholds as follows:

Normal fasting: 70–99 mg/dL (3.9–5.5 mmol/L)
Prediabetes (IFG): 100–125 mg/dL (5.6–6.9 mmol/L)
Diabetes: ≥126 mg/dL (≥7.0 mmol/L) on two separate occasions
Normal 2-h OGTT: <140 mg/dL (<7.8 mmol/L)
Prediabetes 2-h OGTT: 140–199 mg/dL (7.8–11.0 mmol/L)
Diabetes 2-h OGTT: ≥200 mg/dL (≥11.1 mmol/L)
Severe hypoglycemia: <54 mg/dL (<3.0 mmol/L)

These thresholds apply to venous plasma glucose measured in a certified lab after at least 8 hours of fasting. Capillary blood from a home glucometer can read 10–15% higher than venous plasma; CGM interstitial readings can lag venous blood by 5–15 minutes. Diagnosis is never made from a single home reading alone.

When you might need to convert

The most common scenarios where unit conversion matters in everyday life:

International travel with a CGM or glucometer. A US-purchased Dexcom or Libre displays mg/dL by default. Consulting a clinician in the UK, Australia, or Europe — who thinks in mmol/L — requires instant translation. Most apps allow switching units in settings, but not everyone knows to look.
Reading European or Asian clinical studies. The majority of peer-reviewed diabetes research published outside North America reports glucose in mmol/L. If you are following a specific trial protocol or comparing your own values with published cohort data, you need to convert.
Immigrant healthcare transitions. Patients moving to the US from countries using mmol/L — or vice versa — often bring historical glucose logs and A1C records in the "wrong" unit for their new healthcare provider's system.
Online diabetes communities. Forums like r/diabetes and JDRF communities have members globally. A reading of "9.2" (mmol/L) and "9.2 mg/dL" mean radically different things, and the unit is frequently omitted in casual posts.

The quick mental shortcut

When you don't have a calculator handy, the fastest mental conversion is: divide by 18 (or multiply by 18). This approximation is accurate to within 0.1% of the true factor 18.0182, which is far inside any clinically meaningful margin.

A useful set of anchors to memorize:

70 mg/dL ≈ 3.9 mmol/L (low-normal fasting)
100 mg/dL ≈ 5.6 mmol/L (prediabetes threshold)
126 mg/dL ≈ 7.0 mmol/L (diabetes fasting threshold)
180 mg/dL = 10.0 mmol/L (exactly — a useful double-check)
200 mg/dL ≈ 11.1 mmol/L (diabetes 2-h OGTT threshold)

Note that 180 mg/dL = 10.0 mmol/L is mathematically exact because 180 ÷ 18 = 10. This makes it a reliable mental anchor for sanity-checking your conversions.

FAQ

Why do some countries use mg/dL and others mmol/L for blood glucose?

The US adopted mg/dL from an older gravimetric lab tradition; most of the world switched to mmol/L when SI units were standardized in the 1970s–80s. Both measure the same thing — only the scale differs by a factor of 18.0182.

Which countries use mg/dL vs mmol/L?

mg/dL: primarily the United States and some legacy systems in Canada and Latin America. mmol/L: the UK, Australia, New Zealand, most of Europe, India, China, Japan, South Africa, and the majority of countries following SI laboratory conventions.

Are CGM apps like Dexcom and Libre available in both units?

Yes. Dexcom G6/G7, Abbott FreeStyle Libre 2/3, and Medtronic Guardian all support both units in their app settings under "Display Units" or "Glucose Units." Third-party apps like Spike and Nightscout also offer free unit switching.

What is a dangerous blood glucose level?

Severe hypoglycemia (<54 mg/dL / <3.0 mmol/L) is an immediate emergency — impairs cognition and can be life-threatening. The ADA diagnostic threshold for diabetes is ≥126 mg/dL (7.0 mmol/L) fasting. A random reading above 200 mg/dL (11.1 mmol/L) with symptoms is also diagnostic. Always consult your clinician — context, timing, and symptoms determine interpretation.

How do I convert mg/dL to mmol/L without an app?

Divide by 18 (exact factor: 18.0182 — rounding to 18 is accurate to within 0.1%). Example: 126 mg/dL ÷ 18 = 7.0 mmol/L. To reverse, multiply by 18: 5.5 mmol/L × 18 = 99 mg/dL. Memorize the anchor: 180 mg/dL = 10.0 mmol/L exactly.