What are mmol/L and mg/dL?
mmol/L (millimoles per liter) is a unit of molar concentration used in the SI (International System of Units). It expresses the amount of a substance in terms of the number of molecules (or moles) present in one liter of solution. This unit is preferred in most countries worldwide and is considered more scientifically precise because it reflects the actual number of molecules.
mg/dL (milligrams per deciliter) is a unit of mass concentration. It expresses the weight of a substance in milligrams per 100 milliliters (one deciliter) of solution. This unit is primarily used in the United States, and in some contexts in Germany, France, and Japan for certain analytes.
The conversion between these units requires knowing the molecular weight (molar mass) of the substance being measured, since mmol/L is based on molecular count while mg/dL is based on mass.
Conversion Formulas
The factor of 10 accounts for the unit difference between liters and deciliters (1 L = 10 dL) and between grams and milligrams with millimoles.
Common Substances & Molecular Weights
| Substance | Molecular Weight (g/mol) | 1 mmol/L = |
|---|---|---|
| Glucose | 180.16 | 18.016 mg/dL |
| Cholesterol | 386.65 | 38.665 mg/dL |
| Triglycerides | 885.7 | 88.57 mg/dL |
| Creatinine | 113.12 | 11.312 mg/dL |
| Urea / BUN | 28.05 | 2.805 mg/dL |
| Uric Acid | 168.11 | 16.811 mg/dL |
| Calcium | 40.08 | 4.008 mg/dL |
| Bilirubin | 584.66 | 58.466 mg/dL |
| Hemoglobin | 64,458 | 6,445.8 mg/dL |
Unit System Usage by Country
Normal Reference Ranges
| Substance | Normal Range (mmol/L) | Normal Range (mg/dL) |
|---|---|---|
| Fasting Glucose | 3.9 – 5.6 | 70 – 100 |
| Total Cholesterol | < 5.2 | < 200 |
| LDL Cholesterol | < 2.6 | < 100 |
| HDL Cholesterol | > 1.0 (M) / > 1.2 (F) | > 40 (M) / > 50 (F) |
| Triglycerides | < 1.7 | < 150 |
| Creatinine | 0.06 – 0.11 | 0.7 – 1.3 |
| Urea (BUN) | 2.5 – 7.1 | 7 – 20 |
| Uric Acid | 0.15 – 0.45 | 2.5 – 7.5 |
| Calcium | 2.1 – 2.6 | 8.5 – 10.5 |
| Total Bilirubin | 0.003 – 0.021 | 0.2 – 1.2 |
Why Do Countries Use Different Units?
The split between SI units (mmol/L) and conventional units (mg/dL) has historical roots:
- Historical convention: The United States established its clinical laboratory practices before the international push for SI unit standardization in the 1970s. Many US practitioners and systems were already accustomed to mg/dL.
- SI adoption effort: In 1977, the WHO recommended worldwide adoption of SI units for clinical laboratory measurements. Most countries complied, but the US largely did not change for common analytes.
- Clinical familiarity: US clinicians know that a "normal" fasting glucose is around 100 mg/dL. Changing to 5.6 mmol/L would require relearning reference ranges, updating electronic health records, and retraining millions of healthcare workers.
- Scientific accuracy: mmol/L is considered more scientifically meaningful because it reflects the number of molecules reacting in biological processes, not just their weight.
- Mixed systems: Some countries use different units for different tests. For example, many labs worldwide report hemoglobin in g/dL regardless of which system they use for other analytes.
Worked Examples
Example 1: Glucose mmol/L to mg/dL
A patient in the UK has a fasting glucose of 5.5 mmol/L. What is this in mg/dL?
This is within the normal fasting glucose range (70–100 mg/dL).
Example 2: Cholesterol mg/dL to mmol/L
A US patient has a total cholesterol of 240 mg/dL. What is this in mmol/L?
This is above the desirable level (< 5.2 mmol/L) and falls in the high cholesterol range.
Frequently Asked Questions
Why does the conversion factor differ by substance?
Because mmol/L measures the number of molecules and mg/dL measures mass, you need the molecular weight to convert between them. A lighter molecule (like urea, MW 28.05) has a smaller conversion factor than a heavier molecule (like cholesterol, MW 386.65).
Can I just multiply glucose by 18?
Yes, for glucose the shortcut "multiply mmol/L by 18" is widely used and accurate. The exact factor is 18.016 (MW 180.16 / 10), so multiplying by 18 gives a very close approximation.
What about HbA1c?
HbA1c (glycated hemoglobin) uses a different conversion entirely. It is reported as a percentage (DCCT) or mmol/mol (IFCC). The conversion is: DCCT (%) = [0.0915 × IFCC (mmol/mol)] + 2.15%. This is not a simple molecular weight conversion.
Is one unit system more accurate?
Both are equally accurate for clinical use. The mmol/L system is considered more scientifically appropriate because biological reactions depend on molecular interactions (mole-based), not mass. However, the precision of the measurement depends on the assay, not the unit of expression.
Why is hemoglobin's molecular weight so large?
Hemoglobin is a large protein composed of four polypeptide chains, each carrying a heme group. Its molecular weight of approximately 64,458 g/mol reflects this complex quaternary structure. In practice, hemoglobin is typically reported in g/dL or g/L, not mmol/L.