What are SI units in laboratory medicine?

SI units (Système International) are the internationally agreed measurement system used by most of the world for lab results — molar concentrations such as mmol/L, µmol/L and nmol/L. The US mostly uses "conventional" units based on mass, such as mg/dL.

How do you convert a lab value from conventional units to SI units?

Multiply the conventional value by that analyte's conversion factor, which comes from its molecular weight. For example, glucose mg/dL × 0.0555 = mmol/L, and creatinine mg/dL × 88.4 = µmol/L.

Why do different lab tests use different conversion factors?

Because each factor is derived from the molecule's weight. Glucose, cholesterol, creatinine and calcium all have different molecular weights, so they cannot share a single conversion number.

Which countries use SI units for lab results?

Most of Europe, Canada, Australia, and much of Asia report lab values in SI units. The United States mainly uses conventional (mass-based) units, which is why conversions are needed when comparing results across countries or journals.

Are electrolytes reported in SI units?

Electrolytes such as sodium and potassium are usually reported in mmol/L in both systems, so no conversion is needed. Others, like mEq/L values, use ion valence rather than molecular weight.

SI Unit Conversion Calculator – Lab Values SI ↔ Conventional

What Are SI Units?

SI units (the Système International d'Unités) are the internationally standardised way of reporting laboratory results. They express concentration in moles per litre — mmol/L, µmol/L, nmol/L, pmol/L — which reflects the actual number of molecules. Most of Europe, Canada, Australia, and much of Asia report lab results this way.

The United States mostly uses conventional units, based on mass per volume — mg/dL, µg/dL, ng/mL. Both describe the same result; they just count it differently, which is why a conversion factor is needed to move between them.

How the Conversion Works

Each conversion factor comes from the analyte's molecular weight, so every test has its own number:

Conventional to SI SI value = conventional value × conversion factor

SI to conventional Conventional value = SI value ÷ conversion factor

For example, glucose uses 0.0555, so 100 mg/dL × 0.0555 = 5.55 mmol/L. Creatinine uses 88.4, so 1.0 mg/dL × 88.4 = 88.4 µmol/L. Because the factors differ, glucose, cholesterol, and creatinine can never share a single number.

Master SI Conversion Factor Table

Multiply the conventional value by the factor to get the SI value; divide to go back. Where a dedicated calculator exists, it adds reference ranges and clinical context for that test.

Metabolic

Analyte	Conventional	SI	Factor (conv × factor = SI)	Dedicated tool
Glucose	mg/dL	mmol/L	0.0555	Open

Lipids

Analyte	Conventional	SI	Factor (conv × factor = SI)	Dedicated tool
Total cholesterol	mg/dL	mmol/L	0.02586	Open
LDL cholesterol	mg/dL	mmol/L	0.02586	Open
HDL cholesterol	mg/dL	mmol/L	0.02586	Open
Triglycerides	mg/dL	mmol/L	0.01129	Open

Kidney

Analyte	Conventional	SI	Factor (conv × factor = SI)	Dedicated tool
Creatinine	mg/dL	µmol/L	88.4	Open
Urea (BUN)	mg/dL	mmol/L	0.357	Open
Uric acid	mg/dL	µmol/L	59.48	Open

Liver

Analyte	Conventional	SI	Factor (conv × factor = SI)	Dedicated tool
Bilirubin (total)	mg/dL	µmol/L	17.1	Open
Albumin	g/dL	g/L	10	—
Total protein	g/dL	g/L	10	—

Electrolytes

Analyte	Conventional	SI	Factor (conv × factor = SI)	Dedicated tool
Calcium	mg/dL	mmol/L	0.2495	Open
Magnesium	mg/dL	mmol/L	0.4114	—
Phosphate	mg/dL	mmol/L	0.3229	—

Hematology

Analyte	Conventional	SI	Factor (conv × factor = SI)	Dedicated tool
Hemoglobin	g/dL	g/L	10	Open

Iron studies

Analyte	Conventional	SI	Factor (conv × factor = SI)	Dedicated tool
Serum iron	µg/dL	µmol/L	0.179	Open
TIBC	µg/dL	µmol/L	0.179	Open
Ferritin	ng/mL	pmol/L	2.247	Open

Vitamins

Analyte	Conventional	SI	Factor (conv × factor = SI)	Dedicated tool
Vitamin D (25-OH)	ng/mL	nmol/L	2.496	Open
Vitamin B12	pg/mL	pmol/L	0.738	Open
Folate	ng/mL	nmol/L	2.266	Open
Homocysteine	mg/L	µmol/L	7.397	Open

Hormones

Analyte	Conventional	SI	Factor (conv × factor = SI)	Dedicated tool
Testosterone	ng/dL	nmol/L	0.0347	Open
Estradiol	pg/mL	pmol/L	3.671	Open
Cortisol	µg/dL	nmol/L	27.59	Open
TSH	µIU/mL	mIU/L	1	Open
Free T4	ng/dL	pmol/L	12.87	Open
Free T3	pg/mL	pmol/L	1.536	Open

Drugs

Analyte	Conventional	SI	Factor (conv × factor = SI)	Dedicated tool
Digoxin	ng/mL	nmol/L	1.281	Open

SI Units vs Conventional Units: Why Two Systems?

The push to SI units began in the 1970s to make results comparable worldwide and to reflect molecular activity rather than mass. Most of the world adopted them; the US largely kept conventional units in day-to-day practice. The result is that patients, clinicians, and researchers frequently need to translate between the two — for travel, second opinions, or reading international studies.

Common Unit-Pair Converters

If you know the exact units on your report, these focused converters cover the most common pairs:

Frequently Asked Questions

What are SI units in medicine?

Internationally standardised molar units (mmol/L, µmol/L, nmol/L) used for lab results across most of the world.

How do I convert conventional units to SI?

Multiply by the analyte's factor — e.g. glucose mg/dL × 0.0555 = mmol/L.

Why does each test use a different factor?

The factor is set by the molecule's weight, which differs for every analyte.

Do electrolytes need converting?

Sodium and potassium are already mmol/L in both systems; mEq/L values convert using ion valence.

References

UK Kidney Association — Appendix I: Laboratory Conversion Factors (SI ↔ conventional).
Standard clinical chemistry conversion factors derived from analyte molecular weights (AMA Manual of Style; laboratory reference tables).

Conclusion

SI and conventional units describe the same lab result in two different languages — moles versus mass. Pick the analyte above to convert either way, or use the master table as a quick factor reference. For reference ranges and clinical interpretation, open the dedicated calculator for your test.

SI ↔ Conventional Unit Converter