Blood Sugar Converter
Convert blood glucose between mg/dL and mmol/L using the molar mass of glucose, and see how far the rounded factor of 18 moves the answer.
Result
Enter glucose, mg/dl to see the result.
https://calcgallery.com/health-risk/blood-sugar-converter-calculator/
How to use the Blood Sugar Converter
- Check the unit on your meter, sensor app or lab report, mg/dL or mmol/L.
- Choose the direction and enter the reading exactly as shown.
- Read the converted value, and beside it the same conversion with the rounded factor 18.
- Round the result the way your meter does, one decimal for mmol/L or a whole number for mg/dL, if you are comparing with a screen.
Read this first
This is a unit conversion, not medical advice. It changes the units a glucose reading is written in and does nothing else. It cannot tell you whether a reading is in range or what to do about it. Discuss your readings with your clinician.
Where the factor comes from
A concentration in mg/dL says how many milligrams of glucose are in a decilitre of blood. A concentration in mmol/L says how many millimoles are in a litre. Moving between them needs two facts: how much a millimole of glucose weighs, and how many decilitres make a litre.
Glucose is C6H12O6. With the standard atomic weights published by the IUPAC Commission on Isotopic Abundances and Atomic Weights, the same values the molar mass calculator uses, its molar mass is:
M = 6 × 12.011 + 12 × 1.008 + 6 × 15.999 = 180.156 g/molso a millimole weighs 180.156 mg. A litre is 10 decilitres, so:
1 mmol/L = 180.156 mg per 10 dL = 18.0156 mg/dLmmol/L = mg/dL ÷ 18.0156mg/dL = mmol/L × 18.0156Why 18 is used so often
Many meters, labs, conversion charts and clinical papers use a factor of 18. The ADAG study that defines estimated average glucose, for example, gives its mmol/L equation as exactly its mg/dL equation divided by 18. Eighteen is simply the exact factor rounded: easy to do in your head, and close enough that it rarely changes a displayed number.
The difference is a fixed share at every reading, less than a tenth of a percent. Dividing by 18 gives an mmol/L figure that is slightly too high, and multiplying by 18 gives an mg/dL figure that is slightly too low. The page shows both results side by side, and the working shows the size of the gap for your reading.
At the precision readings are displayed, one decimal place in mmol/L and whole numbers in mg/dL, the two factors usually agree. Occasionally a reading sits close enough to a rounding boundary that one factor rounds up and the other down, and the two charts you are comparing disagree by one step in the last digit. Neither is wrong; they rounded a different figure.
Before you compare numbers
Match the units first. A reading of 5.5 is almost certainly mmol/L, and a reading of 99 almost certainly mg/dL. The page warns when a number looks as though it belongs to the other unit.
Compare the same kind of reading. A fingerstick, a continuous sensor and a lab sample measure glucose in different places at different moments. Converting one into the other’s unit does not make them the same measurement.
Only glucose. The factor comes from the molar mass of glucose and is wrong for any other lab result.
With your numbers
- Molar mass of C6H12O6 = 6 × 12.011 + 12 × 1.008 + 6 × 15.999 = 180.156 g/mol
- Factor = 180.156 mg per mmol ÷ 10 dL per litre = 18.0156 mg/dL per mmol/L
- With the rounded factor: 100 ÷ 18 = 5.56 mmol/L, 0.087% higher
- 100 mg/dL ÷ 18.0156 = 5.55 mmol/L
Quick reference
A unit conversion only. What a reading means for you is for your clinician to interpret.
| Example | Result |
|---|---|
| 70 mg/dL | Glucose, mmol/L: 3.89 |
| 100 mg/dL | Glucose, mmol/L: 5.55 |
| 126 mg/dL | Glucose, mmol/L: 6.99 |
| 180 mg/dL | Glucose, mmol/L: 9.99 |
| 250 mg/dL | Glucose, mmol/L: 13.88 |
| 4.0 mmol/L | Glucose, mg/dL: 72.1 |
| 5.5 mmol/L | Glucose, mg/dL: 99.1 |
| 7.0 mmol/L | Glucose, mg/dL: 126.1 |
| 10.0 mmol/L | Glucose, mg/dL: 180.2 |
| 16.7 mmol/L | Glucose, mg/dL: 300.9 |
Where the numbers come from
The fixed figures this calculator relies on, and the published source each one was checked against. How the calculators are built and checked
4 sources
CIAAW, Abridged Standard Atomic Weights 2024 (https://www.ciaaw.org/abridged-atomic-weights.htm): the abridged value that replaces the conventional atomic weight for an interval element, per Prohaska et al., Pure Appl. Chem. 94(5) 573-600 (2022), Table 1 column 7. The standard atomic weight is the interval [12.0096, 12.0116].
Used for
- Conventional atomic weight of carbon; IUPAC publishes the interval [12.0096, 12.0116].
CIAAW, Abridged Standard Atomic Weights 2024 (https://www.ciaaw.org/abridged-atomic-weights.htm): the abridged value that replaces the conventional atomic weight for an interval element, per Prohaska et al., Pure Appl. Chem. 94(5) 573-600 (2022), Table 1 column 7. The standard atomic weight is the interval [1.00784, 1.00811].
Used for
- Conventional atomic weight of hydrogen; IUPAC publishes the interval [1.00784, 1.00811].
CIAAW, Abridged Standard Atomic Weights 2024 (https://www.ciaaw.org/abridged-atomic-weights.htm): the abridged value that replaces the conventional atomic weight for an interval element, per Prohaska et al., Pure Appl. Chem. 94(5) 573-600 (2022), Table 1 column 7. The standard atomic weight is the interval [15.99903, 15.99977].
Used for
- Conventional atomic weight of oxygen; IUPAC publishes the interval [15.99903, 15.99977].
Sacks DB, Arnold M, Bakris GL, et al., "Guidelines and Recommendations for Laboratory Analysis in the Diagnosis and Management of Diabetes Mellitus", Diabetes Care 46(10):e151-e199, 2023, doi:10.2337/dci23-0036, Table 6 footnote: "To convert mg/dL to mmol/L, multiply by 0.0555 or divide by 18." https://pmc.ncbi.nlm.nih.gov/articles/PMC10516260/. Nathan DM, Kuenen J, Borg R, Zheng H, Schoenfeld D, Heine RJ, for the A1c-Derived Average Glucose (ADAG) Study Group, "Translating the A1C assay into estimated average glucose values", Diabetes Care 31(8):1473-1478, 2008, doi:10.2337/dc08-0545, read at https://pmc.ncbi.nlm.nih.gov/articles/PMC2742903/ uses the same factor: its two regressions under Table 2 differ by it (28.7 ÷ 18 = 1.5944 and 46.7 ÷ 18 = 2.594).
Used for
- The rounded factor labs, meters and clinical papers commonly use between glucose in mg/dL and mmol/L. The exact factor is a tenth of the molar mass of glucose, about 18.016.
Common questions about the Blood Sugar Converter
- Is it 18 or 18.016?
- Both are in use. The exact factor is a tenth of the molar mass of glucose, which comes to 18.0156 with today's standard atomic weights. Many meters, labs, clinical papers and conversion charts use 18, which is easier to work with and differs by less than a tenth of a percent. At the precision readings are shown, one decimal in mmol/L or a whole number in mg/dL, the two usually give the same displayed number, and when they do not, they differ by one step in the last digit.
- Why does my meter show a different number from this page?
- If you converted a reading from one unit to the other, a difference in the last digit comes from rounding: the meter, the lab and this page may round at different stages, or use 18 where the page uses 18.0156. A larger difference means the two numbers are not the same measurement: a fingerstick, a continuous sensor and a lab sample taken at different times, or from capillary blood, interstitial fluid and plasma, are expected to differ.
- What range should my blood sugar be in?
- This page does not say, on purpose. What a reading means depends on when it was taken, what was eaten, any medicines and your own situation, and targets are set with a clinician. The page converts the number and leaves its meaning to them.
- Does the same factor work for other lab results?
- No. The factor is specific to glucose because it comes from glucose's molar mass. Cholesterol, triglycerides, creatinine and every other substance have their own molar masses and so their own factors. Using 18 for anything other than glucose gives a wrong answer.
- Can I use this to convert an A1C?
- No. A1C is not a glucose concentration: it is a share of haemoglobin, reported in % or mmol/mol. To see the average glucose that corresponds to an A1C, use the A1C calculator, which applies the ADAG equation.