IU to mg: Why Each Peptide Has Its Own Conversion

Searches for "IU to mg" assume one conversion factor exists. It does not, and that is the useful answer. An international unit is not a quantity of substance; it is an amount of biological activity, defined for one substance at a time by a reference preparation kept by the World Health Organization and measured by an assay. So insulin, growth hormone and oxytocin each have their own factor, set out below from the WHO reference standards' own instructions, and hCG has none at all. This page reports those figures and the arithmetic they allow; it sets no dose.

Mass-to-mass and mass-to-volume conversions are on their own pages — mg to mcg and mg to mL — and the oxytocin case, where a label, a research literature and a vial each use a different unit, is worked through on the oxytocin dosage page.

What an international unit is

An IU defines strength by what a substance does rather than by what it weighs. WHO holds a physical reference — an ampoule of freeze-dried material — and assigns it a number of IU; any other batch is given IU by comparing its activity with that ampoule in an assay. The unit therefore belongs to the substance and to the method, never to a gram.

Two consequences follow. A factor exists only where WHO or a regulator has related the reference's activity to its mass. And where a substance can now be measured by mass, the reference standards have moved to mass. The insulin standard's instructions say its predecessors "were assigned in IU, which no longer reflects the international transition to a mass assigned molecule"; the current insulin and somatropin standards are both assigned in milligrams, with the IU kept as a conversion "if desired", in the words of the somatropin instructions.

The factors that exist

The insulin figure is quoted on the instructions for 11/212 as the "internationally recognized specific activity of pure insulin"; the ampoule itself is assigned 9.19 mg, which that factor turns into the 264.8 IU the instructions report. The somatropin ampoule is assigned 1.95 mg, so at 3.0 IU per mg it is about 5.85 IU. The oxytocin ampoule's mass is given as approximate, so its per-IU figure is approximate too; IU per mg in the table are this site's arithmetic from the stated ratios.

The searched conversions, worked

The spread down the 1 mg row — under 30 IU, 3 IU, nearly 600 IU — is the reason a number in IU means nothing until the substance is named. A label that agrees with the reference figure is the clearest check: the HUMATROPE label lists its somatropin cartridges as 6 mg (18 IU), 12 mg (36 IU) and 24 mg (72 IU), exactly 3 IU per mg. Somatropin and hCG are not calculator compounds on this site and appear here as reference figures only.

hCG: the case with no conversion

The WHO 5th International Standard for chorionic gonadotropin carries two potencies for the same ampoule: 179 IU when it is used to calibrate immunoassays and 162 IU when it is used to calibrate bioassays, a difference of about 10% (arithmetic by this site). Its instructions assign no mass. US hCG labels follow suit: Novarel's vials are "5,000 USP Units" and "10,000 USP Units" and Pregnyl's 10,000 USP units, with the inactive ingredients listed in milligrams and the hormone not. So any page that converts hCG IU to milligrams is supplying a number no reference standard or label gives.

IU are not syringe units

The word "units" on an insulin syringe is the source of a recurring confusion. Those marks measure volume: on a U-100 syringe each one is 0.01 mL, calibrated so that one mark holds one IU of U-100 insulin, which is 100 IU per mL. For insulin at that strength the two meanings coincide. For anything else they do not — a reconstituted peptide at 5 mg/mL puts 0.05 mg in each syringe unit, whatever its IU. Converting a peptide dose to syringe marks is set out on mg to units and reading an insulin syringe; for oxytocin the calculator's oxytocin setting does the arithmetic from milligrams.

What these factors cannot do

A reference factor converts a pure substance measured against the reference. It cannot vouch for a research vial labelled in IU, whose activity no assay on the label confirms, and it cannot carry one substance's ratio over to another, even a close analogue: a modified growth-hormone fragment or a synthetic oxytocin analogue has no IU defined by these standards. Vitamin IU use separate, food-labelling factors and are outside this page. Storage and the general mixing procedure are covered on Medibact's how to store peptides and how to reconstitute peptides pages. Nothing on this page is an instruction; dose belongs with the prescriber.

Sources and dates

Opened 2026-10-02: the NIBSC instructions for use of the WHO 2nd International Standard for insulin, human (11/212), the 2nd International Standard for somatropin (98/574, version 6.0 dated 2025-11-14), the 4th International Standard for oxytocin (76/575) and the 5th International Standard for chorionic gonadotropin (07/364, version 3.0 dated 2013-04-06); the HUMATROPE label via openFDA (effective 2026-08-20), section 3; the NOVAREL (effective 2025-08-27) and PREGNYL (effective 2025-12-02) labels via openFDA, description and dosage sections. Conversions are arithmetic by this site. Corrections go to the contact page.