International Units: Why an IU Is Not a Mass and Cannot Always Be Converted

An International Unit (IU) is a quantity of biological activity, defined by reference to an International Standard held under the authority of the World Health Organization. It is not a quantity of matter, and it is not an SI unit. A conversion from IU to milligrams exists only where two conditions hold: the substance is a single, well-characterised molecule, and every milligram of it carries the same activity. Where the material is a mixture whose composition varies, or where several different molecules share one unit, no fixed factor can be written, and any factor that is quoted applies to one substance, one standard and one method only.

This page sits beside the reference on molar mass and amount of substance. That page converts a mass into a count of molecules; an IU is neither a mass nor a count, and the conversion rules there do not apply until a mass has been established. The examples below are drawn from insulin, the fat-soluble vitamins and heparin, because each shows a different relationship between activity and mass.

What an International Unit is

WHO establishes international biological reference standards to support the standardisation and quality control of biological medicines and diagnostics worldwide. Where appropriate, a standard is assigned a value, for example a potency in International Units; for other standards, no unitage is assigned. Candidate standards are characterised in collaborative studies coordinated by a WHO collaborating centre, and are established on the recommendation of the WHO Expert Committee on Biological Standardization 1.

The unit is therefore a convention attached to a physical object. One ampoule of an International Standard contains an assigned number of IU, and an IU is whatever activity that fraction of the ampoule's contents exhibits in the relevant assay. The WHO recommendations governing these materials set out how they are prepared, filled, characterised for stability, and replaced when stocks run out; a replacement is calibrated against its predecessor so that the unit stays continuous across generations of material 2. Nothing in that chain refers to a mass of the active substance. The unit exists precisely because, when it was created, the active substance could not be weighed in pure form.

How a potency in IU is measured

A sample's potency is determined by comparing its response with the standard's response in the same assay, at several dilutions of each. If the two concentration–response lines are parallel, the horizontal distance between them gives the ratio of potencies, and the sample's potency in IU follows from the standard's assigned value. The assay may be a whole-animal response, a cell-based response, a binding assay or, for well-defined molecules, a physicochemical method calibrated in IU. What matters is that sample and standard behave alike in it.

That requirement is the source of every conversion problem. A potency ratio is only meaningful if the sample and the standard are the same kind of material. A sample with a different composition may respond differently in different assays, and then it has no single potency: its value in IU depends on which assay was used.

Insulin: from an animal response to a fixed mass equivalence

Insulin shows the whole path. The first units, set in Toronto in the early 1920s, were defined by the fall in blood glucose produced in fasted rabbits, because extracts varied widely and there was no other way to state strength. International standards followed, and potency continued to be assigned by bioassay for decades. As purification improved and the molecule could be obtained essentially pure, the activity per unit mass settled to a constant, and the unit became tied to a mass. Knopp and colleagues describe the result: conventional insulin concentrations in IU are based on biological efficacy, SI concentrations are based on mass or amount, and more than one conversion factor between them remains in circulation 4.

The factor now used is stated in both the European and US pharmacopoeias: 1 IU of activity is contained in 0.0347 mg of pure insulin. The 2019 proposal for a new WHO International Standard for human insulin made the change explicit. The candidate material was assigned a content by mass, 9.19 mg per ampoule, with the note that this value can be converted into IU using the internationally recognised specific activity of pure insulin, giving 264.8 IU per ampoule. The document also records why: the previous standard's potency had been defined in IU per milligram, while therapeutic preparations had come to be assayed by HPLC in SI units of mass rather than of bioactivity 3.

The conversion works for insulin because the conditions hold: a single defined sequence, produced to high purity, with a specific activity that does not vary between lots. It applies to pure human insulin. It does not transfer to a modified insulin, whose molecular mass and activity per mass differ; each such product carries its own relation between labelled units and mass, stated in its own documentation.

Vitamins D and A: a unit defined as a mass, and its retirement

The fat-soluble vitamins took the opposite route. Their International Units were fixed as the activity of a specified mass of a single reference compound, so the conversion is exact by definition. For vitamin D, 1 µg is equal to 40 IU, and 1 IU is therefore 0.025 µg 6. Both scales describe the same quantity of the same molecule; the IU survives on labels mostly as habit.

Vitamin A shows what happens when several molecules share one unit. Preformed retinol and the provitamin carotenoids all contribute vitamin A activity, but with different efficiencies, and those efficiencies differ again between dietary and supplemental sources. A single IU could not describe them all at a fixed factor. The unit was replaced by retinol activity equivalents (RAE): 1 µg RAE corresponds to 1 µg of retinol, 2 µg of supplemental beta-carotene, 12 µg of dietary beta-carotene, or 24 µg of dietary alpha-carotene or beta-cryptoxanthin. For retinol itself, 1 IU equals 0.3 µg RAE, and US labelling has moved from IU to µg RAE 7.

The general lesson is that an IU applied across different chemical forms can only be converted form by form. A total expressed in IU for a mixture of forms cannot be turned into a single mass without knowing the proportions.

Heparin: a mixture with no mass equivalence

Heparin is a polydisperse mixture of sulfated polysaccharide chains extracted from animal tissue. Its anticoagulant activity depends on the distribution of chain lengths and on specific binding sequences present in only part of the material, both of which vary between sources and lots. There is no single molecule to weigh and no constant activity per milligram, so heparin is labelled and dispensed by activity and has never had a universal mass conversion.

Heparin also shows that units can drift. Over about three decades, the US Pharmacopeia unit and the WHO International Unit for unfractionated heparin diverged by roughly 10%, the USP unit reading higher. A revised USP monograph, effective in October 2009, introduced a new reference standard and a new test for impurities and harmonised the USP unit with the WHO International Standard. The result was that material labelled with a given number of USP units after the change carried about 10% less activity than material labelled with the same number before it. The US regulator asked manufacturers to delay shipments made under the new monograph so that users could adjust 5.

No mass figure was involved at any stage. Two activity scales anchored to two different reference materials had separated, and the correction was made by realigning one standard with the other.

When a conversion factor can exist

Bench rules

1. Record activity exactly as stated, in the unit stated, with the standard it refers to where given.
2. Do not convert IU to mass without a published factor, and record the factor, its source and the substance it was published for.
3. Apply a factor only to the substance it was published for. A factor for one molecule says nothing about a modified form or a mixture.
4. Convert to amount of substance only after a mass has been established, then follow the molar mass rules for that entity.
5. Where a certificate states both activity and mass, check that they agree with the published factor. A discrepancy points to impurity, degradation or a different standard.
6. When an International Standard is replaced, note the change against any long-running series of results; continuity is intended but must be checked.

An IU carries information a mass cannot: how much of the intended activity is present. That is its value, and it is also why it resists conversion. A conversion factor is a finding about one pure substance, not a property of the unit, and it should be treated with the same care as any other measured value 2.