| Analyte | Conventional | → SI | Factor |
|---|---|---|---|
| Glucose | mg/dL | mmol/L | ÷ 18.0 |
| Total cholesterol, LDL, HDL | mg/dL | mmol/L | ÷ 38.67 |
| Triglycerides | mg/dL | mmol/L | ÷ 88.57 |
| Urea nitrogen (BUN) | mg/dL | mmol/L | ÷ 2.80 |
| Creatinine | mg/dL | µmol/L | × 88.4 |
| Bilirubin | mg/dL | µmol/L | × 17.1 |
| Calcium | mg/dL | mmol/L | ÷ 4.0 |
| Uric acid | mg/dL | mmol/L | ÷ 16.81 |
| Albumin | g/dL | g/L | × 10 |
| Haemoglobin | g/dL | g/L | × 10 |
| Iron | µg/dL | µmol/L | ÷ 5.587 |
| Vitamin D (25-OH) | ng/mL | nmol/L | × 2.496 |
| Vitamin B12 | pg/mL | pmol/L | × 0.738 |
| Ferritin | ng/mL | µg/L | × 1 (same) |
| TSH | µIU/mL | mIU/L | × 1 (same) |
Why every factor is different
Conventional units are a mass per volume. SI units are a count of molecules per volume.
Converting means dividing by the molecular weight, and every substance has its own. Glucose is 180 g/mol, cholesterol 386.7, triglycerides around 885. Heavier molecule, larger divisor.
That is the whole explanation, and it is why any page offering a single conversion factor for "lab values" is wrong before it starts.
Note the two rows at the bottom where the factor is 1. Ferritin and TSH already use numerically identical units in both systems — the names differ and the number does not.
Watch the direction on creatinine and bilirubin
Most conversions on this page divide going from conventional to SI. Creatinine, bilirubin and vitamin D multiply, because the SI unit is a thousandfold smaller (µmol rather than mmol).
Getting the direction wrong on creatinine produces a value roughly 7,800 times off, which is at least obvious. Getting it wrong on a value where the factor is close to 1 is much easier to miss.
Worked examples
Glucose 126 mg/dL ÷ 18 = 7.0 mmol/L — the fasting diabetes threshold in both systems.
LDL 100 mg/dL ÷ 38.67 = 2.6 mmol/L — a common target.
Creatinine 1.0 mg/dL × 88.4 = 88.4 µmol/L.
Vitamin D 30 ng/mL × 2.496 = 75 nmol/L — the sufficiency threshold in both.
Converting a value does not convert its interpretation
This is the part that matters more than the arithmetic.
A reference range comes from the laboratory that ran the sample, and ranges differ between laboratories because assays and reference populations differ. A converted number compared against a range from a different laboratory stacks two sources of error.
Where a result matters, compare it with the range printed on the same report.
Where the two systems are actually used
SI units are standard across most of Europe, Australia, New Zealand, Canada and much of Asia. Conventional units remain standard in the United States, and in a handful of other places for specific analytes.
The practical consequence for anyone reading medical literature or comparing results across borders: guidelines quote thresholds in their local system, and a threshold that looks unfamiliar is usually the same one expressed differently. A fasting glucose cut-off of 7.0 and one of 126 are not two schools of thought; they are one number twice.
The same is true of the round-number problem in reverse. A threshold that is memorably round in one system is awkward in the other, and the awkward version is the converted one. That is a quick way to tell which system a guideline was written in.
A1C is not on this list, deliberately
The A1C-to-average-glucose relationship is an empirical fit from a clinical study, not a unit conversion — different in kind from everything above, and it has its own page.
Body temperature conversion is a linear transform rather than a molecular one, and it is handled separately.
Sources: Clinical Chemistry SI unit conversion tables; NIH laboratory reference material; AMA Manual of Style SI conversion appendix.
Reference tables for general information, not medical advice. Interpret any result with the range on the report and with the clinician who ordered it.