Laboratory Measurement Units: mg, mcg, and IU

Milligrams (mg), micrograms (mcg or µg), and International Units (IU) do not all describe the same property. Correct interpretation matters when researchers compare a label, certificate of analysis, analytical method, and study record.

Research Use Only. This article explains measurement terminology for qualified laboratory personnel. It is not intended for human or veterinary use.

Milligrams and micrograms describe mass

The milligram and microgram are decimal submultiples of the gram. The relationship is:

1 mg = 1,000 mcg

Unit symbols are case-sensitive. A value should always be transcribed with its unit, source document, and significant figures so that a conversion remains auditable.

Concentration includes a denominator

A result such as mg/mL or mcg/mL describes mass concentration: a stated mass per unit volume. It is different from the total mass listed for a container. In analytical records, concentration can be represented as c = m / V, provided the mass and volume refer to the same defined material and record. Laboratories should document the source value, units, calculation, and responsible reviewer under their approved SOP.

International Units describe assigned biological activity

An International Unit is assigned through a substance-specific reference standard or method. It is not a universal unit of mass. An IU value for one biological material cannot be converted to milligrams by borrowing a factor from another material. Any relationship between activity and mass must come from the applicable reference standard, manufacturer documentation, or validated analytical method.

Reading a COA correctly

• Identity asks whether the submitted sample is the stated compound.
• Purity reports the result of the named analytical method, often as a percentage.
• Content or quantity describes an amount and is not automatically proven by a chromatographic purity percentage.
• Activity may be reported in a method- or reference-specific unit such as IU.

Where a purity specification applies, it is a product-specific target and not a substitute for a lot-specific result. Researchers should review the independent laboratory report for the applicable product and lot before relying on a stated purity.

References

• National Institute of Standards and Technology: SI Units — Mass.
• World Health Organization: International biological reference standards.

For in-vitro laboratory research and analytical use only. Not for human or veterinary use.