mmol/L vs mg/dL: how to convert blood sugar readings
This article is general information, not medical advice. It is not a substitute for a doctor, dietitian or other qualified professional who knows your circumstances. If you have a medical condition, are pregnant, or are changing your diet or exercise significantly, talk to one first.
To convert blood sugar from mg/dL to mmol/L, divide by 18; to go the other way, multiply by 18. So 100 mg/dL is 5.6 mmol/L, and 7.0 mmol/L is 126 mg/dL. Both units measure exactly the same thing, the glucose in your blood, just counted differently. The blood sugar converter does the sum in both directions and places the reading against standard ranges; this guide explains the numbers behind it.
The conversion
mg/dL counts the mass of glucose: milligrams in every decilitre (100 ml) of blood. mmol/L counts the number of molecules: millimoles in every litre. One mole of glucose weighs about 180 grams, so 1 mmol/L is about 180 mg per litre, which is 18 mg per decilitre. The precise factor is 18.016, which is what the converter uses, but dividing by 18 is accurate enough for everyday use. Round mmol/L to one decimal place and mg/dL to a whole number; no home meter is precise enough to justify more.
A few anchors make mental conversion easy: 4 mmol/L is about 72 mg/dL, 5.5 is about 99, 7 is 126, 10 is 180 and 11.1 is 200.
Why two units exist
Most of the world reports lab results in SI units, which measure substances in moles; for glucose that means mmol/L, the unit used in the UK, Canada, Australia and many other countries. The United States kept mass-based units, so American meters, labs and guidelines use mg/dL, and several other countries do the same. The split matters when you move abroad, read a guideline written elsewhere or buy a meter online: a reading of 6.0 on a mmol/L meter and 108 on a mg/dL meter are the same sugar level. Some meters can be switched between units and others are fixed for the market they were sold in, so check the unit on the display before comparing numbers.
Key numbers in both units
- Fasting, normal: below 100 mg/dL (5.6 mmol/L).
- Fasting, prediabetes range: 100 to 125 mg/dL (5.6 to 6.9 mmol/L) in US guidelines. The WHO starts its impaired fasting glucose range a little higher, at 110 mg/dL (6.1 mmol/L).
- Fasting, diabetes range: 126 mg/dL (7.0 mmol/L) or higher, confirmed by repeat testing.
- Two hours after a 75 g glucose drink: below 140 mg/dL (7.8 mmol/L) is normal, 140 to 199 (7.8 to 11.0) is impaired glucose tolerance, and 200 (11.1) or more is in the diabetes range.
- Low (hypoglycaemia): below 70 mg/dL (3.9 mmol/L) is generally treated as low, and the NHS uses below 4 mmol/L. Below 54 mg/dL (3.0 mmol/L) is classed as significantly low.
These are diagnostic and safety thresholds. People living with diabetes usually have personal targets agreed with their care team, which can differ from the numbers above.
HbA1c has its own units
HbA1c is a blood test that reflects your average glucose over the previous two to three months, and it also comes in two units: a percentage, common in the US, or mmol/mol, used in the UK and many other countries. A result of 6.5% equals 48 mmol/mol, the threshold commonly used to diagnose diabetes, and 5.7% is about 39 mmol/mol. Converting between these is not the same as converting glucose; it uses its own formula, which the A1c calculator applies, along with an estimate of the average glucose that an HbA1c result corresponds to.
Mistakes to avoid
- Mixing units in one log. A reading of 5.8 and one of 105 are the same level; put both in one column and the log stops making sense.
- Converting twice. If an app already converts for you, converting its output again gives nonsense.
- Expecting meters to match the lab. Standards allow home meters a margin of error of roughly 15%, and results vary with technique. Diagnosis uses lab tests.
- Forgetting the lag. Continuous glucose monitors measure fluid under the skin, which trails blood by several minutes, especially when glucose is changing quickly.
- Ignoring context. A number means little without knowing whether it was fasting, after a meal or random.
When a reading needs action
A single high finger-prick reading is not a diagnosis: diabetes is diagnosed with lab tests, usually repeated on another day. But some readings need prompt action. If you use insulin or another medicine that can cause hypos and your reading is below 4 mmol/L (about 70 mg/dL), follow your hypo plan straight away with fast-acting sugar, then recheck. If someone is confused, very drowsy or unconscious, call emergency services and do not give food or drink to anyone who cannot swallow safely. Persistently high readings, especially with thirst, passing a lot of urine, weight loss or vomiting, need same-day medical advice. If you are tracking other numbers for a check-up, What is a normal blood pressure? How to read the numbers explains blood pressure readings, and the blood pressure log keeps them in one place. This guide is general information, not medical advice.
Sources and further reading
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