About the Blood Sugar Converter
Blood glucose is measured in two units, and which one you see depends mostly on where you live. The United States, and a few other countries, use milligrams per decilitre (mg/dL). The UK, Canada, Australia and most of Europe use millimoles per litre (mmol/L).
The conversion between them is simple arithmetic, but a converted number on its own does not tell you much. A reading of 130 mg/dL is perfectly ordinary two hours after lunch and in the diabetes range first thing in the morning before breakfast.
So this blood sugar converter does two jobs. It converts the reading using the exact factor, and it places it against the right thresholds for when the reading was taken — fasting, two hours after a meal, or at a random time — as well as against the limits for low blood sugar.
How to Use the Blood Sugar Converter
Choose the unit your reading is in, enter the number, and say when it was taken.
Fasting means at least eight hours without food or caloric drinks, usually first thing in the morning.
Two hours after eating is timed from the start of the meal.
Random covers any other time.
How the Conversion Works
mmol/L = mg/dL ÷ 18.016
mg/dL = mmol/L × 18.016The factor is not a convention anyone chose. It falls out of the chemistry.
A mole of glucose weighs 180.16 grams. A concentration in mg/dL is a tenth of the same concentration in mg/L, and dividing mg/L by the molar mass gives mmol/L. Put together, the factor is 180.16 ÷ 10 = 18.016.
The familiar "divide by 18" is this figure rounded. The difference is less than a tenth of one per cent, which never changes a result in any way that matters.
Step-by-Step Example
Fasting reading of 126 mg/dL.
126 ÷ 18.016 = 6.99 → 7.0 mmol/LAgainst fasting thresholds, 126 mg/dL is exactly where the diabetes range begins, and 7.0 mmol/L is the matching published threshold in the other unit. A single reading like this is a prompt to test properly, not a diagnosis.
Thresholds by Situation
| When taken | Normal | Prediabetes | Diabetes |
|---|---|---|---|
| Fasting | under 100 mg/dL (5.6) | 100–125 (5.6–6.9) | 126 or more (7.0) |
| 2 hours after eating | under 140 (7.8) | 140–199 (7.8–11.0) | 200 or more (11.1) |
| Random | — | — | 200 or more (11.1), with symptoms |
Low blood sugar applies at any time: under 70 mg/dL (3.9 mmol/L) is hypoglycaemia, and under 54 mg/dL (3.0 mmol/L) is clinically significant.
Why the Thresholds Do Not Convert Exactly
Look closely and the two columns do not quite line up.
126 mg/dL is 6.99 mmol/L, published as 7.0. 100 mg/dL is 5.55 mmol/L, published as 5.6. 70 mg/dL is 3.89 mmol/L, published as 3.9.
Each unit's thresholds were rounded to a convenient figure on their own. So a reading that sits right on a boundary can fall just inside it in one unit and just outside in the other. The converter uses each unit's published threshold as published, rather than converting one set into the other and pretending the match is exact.
Meters, Labs and Diagnosis
Home meters are good, but they are not laboratory instruments. The international standard for meters allows readings within about 15% of a laboratory value (or 15 mg/dL at low levels) for 95% of results. That is plenty for adjusting insulin or spotting a pattern, and not enough to diagnose anything.
Diabetes is diagnosed from laboratory tests — a fasting plasma glucose, an oral glucose tolerance test, or an A1C — repeated on a different day, unless someone has obvious symptoms with a very high random reading.
Readings also vary for ordinary reasons: stress, illness, poor sleep, a hard workout or a late meal the night before can all move a fasting reading.
Low Blood Sugar
Hypoglycaemia mostly affects people taking insulin or certain tablets that make the pancreas release insulin, such as sulfonylureas.
The usual approach for a reading under 70 mg/dL (3.9 mmol/L) is the 15–15 rule: take 15 grams of fast-acting sugar — glucose tablets, a small glass of juice or regular soft drink — then recheck after 15 minutes and repeat if still low. Follow with a snack or meal once it has come up.
Anyone who is drowsy, confused, having a seizure or unable to swallow safely needs emergency help, not food by mouth.
Understanding Your Result
Converted is your reading in the other unit.
Both units shows the pair side by side, with mmol/L to one decimal place and mg/dL as a whole number, matching how meters display them.
What it means places the reading against the thresholds for when it was taken, or against the limits for low blood sugar.
Thresholds lists the published figures in both units.
Worth knowing covers what to do about low or very high readings, and why one reading is not a diagnosis.
When Should You Use This Calculator?
When reading research, food labels or advice written in the other unit.
When travelling or moving between countries that use different units.
When sharing readings with a clinic that uses a different system.
Not to diagnose diabetes, and not in place of urgent care for very low or very high readings with symptoms.
Common Mistakes
Judging an after-meal reading against fasting limits. The thresholds are different for a reason.
Checking too soon after eating. An hour after a meal is naturally higher than two.
Mixing up mmol/L and mmol/mol. The first is glucose; the second is A1C.
Treating one meter reading as a diagnosis. Meters have a margin; diagnosis needs lab tests.
Assuming the thresholds convert exactly. Each unit's figures were rounded separately.
If you have diabetes, your own targets may differ from these general thresholds, and your care team is the right source for them. Nothing here is medical advice.