mmol/L to mg/dL: converting blood glucose and A1c units

What mmol/L and mg/dL measure

Both units describe concentration, meaning how much glucose is dissolved in a set volume of blood. They differ in how the glucose is counted.

• mg/dL (milligrams per decilitre) is a mass concentration. It’s the weight of glucose, in milligrams, in each decilitre of blood. A decilitre is a tenth of a litre, or 100 mL.
• mmol/L (millimoles per litre) is a molar concentration. A mole is a fixed, very large number of molecules, and a millimole is a thousandth of a mole. So mmol/L tells you how many glucose molecules, counted in millimoles, are in each litre.

One unit weighs the glucose and the other counts it. A roll of identical coins can be described by its weight or by how many coins it holds, and if you know what one coin weighs, either number gives you the other. For glucose, that weight is the molar mass, the mass of one mole of molecules.

Where the factor of 18 comes from

A glucose molecule is made of 6 carbon, 12 hydrogen and 6 oxygen atoms (C6H12O6). Its molar mass is 180.16 grams per mole, and the US National Institute of Standards and Technology lists it as 180.1559 ¹. The conversion then takes three steps.

1. One mole of glucose weighs 180.16 g, so one millimole weighs 180.16 mg.
2. A concentration of 1 mmol/L therefore means 180.16 mg of glucose in each litre.
3. A litre holds 10 decilitres, so each decilitre contains 18.016 mg.

That makes 1 mmol/L equal to 18.016 mg/dL, which gives two formulas.

• mg/dL = mmol/L × 18.016
• mmol/L = mg/dL ÷ 18.016 (the same as multiplying by 0.0555)

For example, 7.0 mmol/L × 18.016 = 126.1, so 7.0 mmol/L is 126 mg/dL. In the other direction, 100 mg/dL ÷ 18.016 = 5.55, which rounds to 5.6 mmol/L.

You’ll often see the factor rounded to 18. That changes the result by less than 0.1%, which moves the rounded answer by 1 mg/dL at most. For example, 7.8 mmol/L is 140.4 mg/dL with 18 and 140.5 with 18.016. The calculations in this article use 18.016.

To convert in your head, multiply mmol/L by 2 and then by 9 (7 × 2 = 14, and 14 × 9 = 126). For the reverse, halve the mg/dL value and divide by 9 (180 ÷ 2 = 90, and 90 ÷ 9 = 10). The factor works only for glucose. Cholesterol and other substances have molecules of different weights, so each has its own factor.

In plain terms: Multiply mmol/L by 18 to get mg/dL. Divide mg/dL by 18 to get mmol/L. Using the exact factor, 18.016, moves the rounded answer by 1 mg/dL at most.

Who uses which unit

Canada works in mmol/L. Diabetes Canada’s clinical practice guidelines give glucose values in mmol/L. To reach an A1c of 7.0% or less, they suggest aiming for 4.0-7.0 mmol/L when fasting or before meals and 5.0-10.0 mmol/L two hours after eating ². In 2010, Health Canada told manufacturers that a blood glucose meter applying for a Canadian licence had to report results only in mmol/L ³.

The United States works in mg/dL. Guidance from the US Food and Drug Administration (FDA) on home blood glucose meters says meters intended for use in the US should report results in mg/dL ⁴. The American Diabetes Association (ADA) Standards of Care put mg/dL first, with mmol/L in parentheses. In the 2024 edition, the before-meal goal for many nonpregnant adults is 4.4-7.2 mmol/L (80-130 mg/dL) ⁵.

Those two before-meal goals differ in more than their units. Diabetes Canada’s 4.0-7.0 mmol/L works out to about 72-126 mg/dL, while the ADA’s range starts and ends higher ^{2, 5}. When numbers from two countries don’t line up, convert first, then check whether the targets themselves differ.

International consensus documents often print both units. The 2019 international consensus on time in range writes mg/dL first, with mmol/L in parentheses ⁶. A 2017 joint position statement of the ADA and the European Association for the Study of Diabetes (EASD) put mmol/L first, starting with its title, which calls for glucose values below 3.0 mmol/L (54 mg/dL) to be reported in clinical trials ⁷.

Where the standard cut-offs come from

The cut-offs used for time in range were agreed in consensus reports published in 2017. In one, a steering committee representing organizations that included the ADA, the Endocrine Society and JDRF defined outcome measures for type 1 diabetes beyond A1c ⁸.

• Level 1 hypoglycemia: below 3.9 mmol/L (70 mg/dL) and at or above 3.0 mmol/L (54 mg/dL)
• Level 2 hypoglycemia: below 3.0 mmol/L (54 mg/dL)
• Level 1 hyperglycemia: above 10.0 mmol/L (180 mg/dL), up to 13.9 mmol/L (250 mg/dL)
• Level 2 hyperglycemia: above 13.9 mmol/L (250 mg/dL)
• Time in range: the share of readings within 3.9-10.0 mmol/L (70-180 mg/dL)

The joint ADA and EASD statement from the same year described a glucose of 3.9 mmol/L (70 mg/dL) or less as an alert value, and a level below 3.0 mmol/L (54 mg/dL) as low enough to signal serious, clinically important hypoglycemia ⁷. The 2019 time-in-range consensus built its targets on the same cut-offs ⁶, and the ADA’s classification of hypoglycemia reprints the 2017 definitions ^{5, 8}. For pregnancy, the time-in-range consensus uses a narrower range of 3.5-7.8 mmol/L (63-140 mg/dL) ⁶.

Values such as 70, 180 and 250 mg/dL are round numbers. Divided by 18.016, 70 and 250 become 3.885 and 13.877 mmol/L, which round to 3.9 and 13.9. Two pairs line up neatly in both units, because 3 × 18 = 54 and 10 × 18 = 180.

The table lists values you’ll meet often. The third column shows the exact product of the mmol/L value and 18.016.

The levels and ranges come from the consensus documents above ^6–8. The pregnancy goals are Diabetes Canada’s for people whose diabetes began before pregnancy ⁹. The diagnostic cut-offs, also Diabetes Canada’s, apply to venous blood measured in a laboratory, and without clear symptoms a result in the diabetes range needs a repeat lab test on another day ¹⁰.

For 6.7 and 7.8 mmol/L, the exact products round to 121 and 141 mg/dL. The usual pairs work from the other direction. Starting from 120 and 140 mg/dL gives 6.66 and 7.77 mmol/L, which round to 6.7 and 7.8.

Why 70 mg/dL becomes 3.9 mmol/L

Glucose in mmol/L is normally written to one decimal place and mg/dL as a whole number, which is how the consensus documents print them ^{6, 7}. Converting 70 mg/dL gives 3.885 mmol/L, and rounding to one decimal gives 3.9. Writing 3.89 would suggest a precision that home meters don’t have. FDA guidance asks that 95% of a meter’s results fall within 15% of the results from a comparison method across its measuring range ⁴. At 3.9 mmol/L, 15% is about 0.6 mmol/L (10.5 mg/dL), far larger than any rounding.

Rounding explains a few small mismatches you may notice.

• Steps differ in size. A step of 0.1 mmol/L equals 1.8 mg/dL, so one decimal in mmol/L is slightly coarser than a whole number in mg/dL. A value rounded to 3.9 mmol/L could be anything from 3.85 to just under 3.95 mmol/L, about 69 to 71 mg/dL.
• Round trips can drift. Converting 120 mg/dL gives 6.7 mmol/L, and converting 6.7 mmol/L back gives 121 mg/dL.
• Range edges follow the steps. The time-in-range consensus writes level 1 low as 3.0-3.8 mmol/L (54-69 mg/dL) and level 1 high as 10.1-13.9 mmol/L (181-250 mg/dL) ⁶. Converted, 181 mg/dL is 10.05 mmol/L, and the first one-decimal value above 10.0 is 10.1.

These differences are smaller than the measurement error that meter standards allow ⁴.

A1c has two units of its own

A1c, the share of hemoglobin with glucose attached, is reported in two units that come from two measurement systems.

Canadian guidelines and US laboratories use percent ^{2, 11}. Through the National Glycohemoglobin Standardization Program (NGSP), the percent scale is tied to A1c results from two landmark trials, the Diabetes Control and Complications Trial (DCCT) and the UK Prospective Diabetes Study (UKPDS). Care guidelines from the ADA and other diabetes organizations relate directly to these numbers ¹¹.

The mmol/mol unit is based on a reference method of the International Federation of Clinical Chemistry and Laboratory Medicine (IFCC), published in 2002. It defines A1c as hemoglobin with glucose stably attached at one exact spot, the start of its beta chain. The method cuts hemoglobin into fragments with an enzyme and measures the ratio of glycated to non-glycated fragments from that spot. Because it is more specific, it gives lower results than most routine methods in use at the time ¹². In 2007, the IFCC recommended reporting the result as millimoles of A1c per mole of hemoglobin, or mmol/mol ¹¹.

The master equation

Between 2001 and 2003, the IFCC reference laboratories and the US, Japanese and Swedish standardization networks measured the same pooled blood samples in four studies. The relationship between IFCC and NGSP results was a straight line that didn’t change significantly from one study to the next, published as NGSP (%) = 0.915 × IFCC (%) + 2.15 ¹³. That paper gave IFCC results as a percentage, and multiplying by 10 turns them into mmol/mol ^{11, 13}. Rearranged, the equation gives two conversions.

• IFCC (mmol/mol) = (NGSP % − 2.15) × 10.929
• NGSP (%) = 0.0915 × IFCC (mmol/mol) + 2.15

The NGSP states the master equation with one more digit, NGSP = 0.09148 × IFCC + 2.152 with IFCC in mmol/mol, and writes the reverse as IFCC = 10.93 × NGSP − 23.50 ¹¹. For the values in the table below, every version gives the same whole number.

Two things follow. A1c in mmol/mol isn’t the percent times 10, since 7.0% is 53 mmol/mol. And each percentage point is worth about 11 mmol/mol, so a fall from 8.0% to 7.0% is a fall from 64 to 53 mmol/mol.

Values are calculated with IFCC = (NGSP − 2.15) × 10.929 and rounded to whole numbers.

In 2010, a consensus committee of the ADA, the EASD, the International Diabetes Federation, the IFCC and the International Society for Pediatric and Adolescent Diabetes recommended that laboratories worldwide report A1c in both units, giving mmol/mol with no decimals and percent with one decimal ¹⁴. Practice varies. Diabetes Canada uses percent, with 6.5% (48 mmol/mol) or more as one of its criteria for diabetes, 6.0-6.4% (42-46 mmol/mol) as one marker of prediabetes ¹⁰ and 7.0% (53 mmol/mol) or less as the target for most people with type 1 or type 2 diabetes ². The ADA writes percent with mmol/mol in parentheses ⁵, and some countries report mmol/mol alone or alongside percent ¹¹.

How meters and apps set the unit

In North America, the unit a meter shows depends on where it’s sold. FDA guidance says meters for use in the US should report results in mg/dL, as plasma-equivalent values ⁴. Health Canada’s 2010 notice went further. A meter applying for a Canadian licence had to report only in mmol/L, with a unit that neither the user nor a drop or battery change could alter, and any software supplied with it had to show mmol/L only ³.

Those rules followed real problems. Health Canada noted that the unit shown on some meters could change by accident when the device was dropped, when batteries were changed or with no apparent cause ³. In 2005, Abbott recalled several meter models in the US because users could switch the unit from mg/dL to mmol/L without meaning to. According to the FDA’s recall record, results could then be misread, which could lead to undertreatment and high glucose. The company’s fix included locking the unit on newly made meters ¹⁵.

Phone and CGM apps display a unit as well, so the same check applies to them. And because US and Canadian meters are set up for different units, a meter bought in another country may show a different one.

When units get mixed up

Because the two scales differ by a factor of about 18, a number read in the wrong unit is far off. A reading of 100 is ordinary in mg/dL (5.6 mmol/L) and would be extremely high in mmol/L. A reading of 5.6 is ordinary in mmol/L and would be dangerously low in mg/dL. Health Canada warned that a person who overestimates their glucose might take too much insulin and have a severe low, with loss of consciousness or a seizure ³. The recall above shows the reverse risk, undertreatment and high glucose ¹⁵.

Published data on how often unit mix-ups happen, or how often they cause harm, are scarce. The evidence above comes from regulators’ notices and a recall, which describe the problem without counting cases. The practical lessons are simple.

• Check the unit label when a reading looks far off, and whenever you start using a new, borrowed or imported meter.
• Convert before you compare targets or results from another country’s guidelines, studies or reports.
• If a converted number doesn’t match what your care team gave you, ask them before changing anything in your treatment.