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Somatropin is labelled in milligrams on some products and in international units on others. One milligram is 3.0 IU, a figure that comes from the WHO International Standard rather than from convention. When both the vial and the dose are stated in IU the conversion is not needed at all — the units cancel.

Last reviewed: September 21, 2026

Somatropin is labelled in milligrams or in international units. 1 mg is 3.0 IU by the WHO international standard.

Optional. Gives the same mass in international units, at the WHO somatropin standard's 3.0 IU per mg.

How the arithmetic works

Somatropin (recombinant human growth hormone) is labelled in milligrams on current US prescribing information, and historically — and in some vials sold today — in international units (IU). Converting between the two:

  • IU = mg × 3.0
  • mg = IU ÷ 3.0

Reading a dose onto a U-100 syringe, for a vial and dose both stated in the same unit, does not need this conversion at all — the unit cancels:

  • Syringe units = (dose ÷ vial strength) × water volume (mL) × 100

Source and its caveat. 3.0 IU per mg is the WHO 2nd International Standard for Somatropin’s own assigned value (NIBSC code 98/574, Instructions for Use Version 6.0, 14 November 2025), which states: “3.0 International units per mg Somatropin.” The same document adds that this figure “relates to somatropin monomer, which should again be independently determined” — it is the standard’s defined conversion, not a guarantee that any specific vial’s full labelled content is monomer in that proportion. Current FDA-regulated somatropin labels (Genotropin’s, for instance) are written in mg only and do not use IU at all.

Worked examples

Converting a mass: 1 mg of somatropin —

Mass to units1 mg × 3.0 IU/mg = 3 IU

Reading a dose on a syringe: a 36 IU vial reconstituted with 1 mL of water, for a 2 IU dose — entirely in IU, so 3.0 IU/mg plays no part in this calculation:

Vial in milligrams36 IU ÷ 3.0 IU/mg = 12 mg — shown for reference only
Draw volume(2 IU ÷ 36 IU) × 1 mL = 0.0556 mL
Syringe units0.0556 mL × 100 = 5.56 units

Enter the same numbers into the calculator above and it returns the same figures.

What changes the answer

  • Which unit the vial and dose are stated in. When both are the same unit (both mg, or both IU), the syringe-reading arithmetic never touches the 3.0 IU/mg conversion. It matters only when converting a mass figure to or from IU directly, or when a vial and a dose are stated in different units from each other.
  • Water volume. More water lowers the vial’s concentration, raising the unit count for the same dose; less water does the reverse — the same relationship as any other reconstituted vial.

FAQ

How many IU is 1 mg of HGH?
3.0 IU, at the WHO 2nd International Standard for Somatropin's assigned conversion (NIBSC 98/574). That figure relates to somatropin monomer specifically — the standard's own instructions for use state that the conversion should be independently determined for a given preparation's actual monomer content, since finished products can include higher-molecular-weight forms and related impurities alongside the monomer.
hgh mg to iu conversion
Multiply milligrams by 3.0 to get international units, or divide IU by 3.0 to get milligrams — the WHO somatropin standard's assigned conversion. This page's calculator performs either direction, and separately reads a dose on a U-100 syringe from a vial's IU strength directly, which does not require this conversion at all.
Why doesn't my HGH label show IU?
Current FDA-regulated somatropin labels are written in milligrams only. Genotropin's own DailyMed prescribing information, for instance, does not contain the letters "IU" anywhere in the document — IU is a historical unit this page's mg↔IU conversion exists to bridge, not something every current label states.
Do I need the mg-to-IU conversion if my vial is already labelled in IU?
No. When both the vial's strength and the dose are in IU, the unit cancels out of the syringe-reading arithmetic entirely — units = (dose IU ÷ vial IU) × water volume × 100 — so the 3.0 IU/mg figure never enters the calculation. It matters only for converting a mass in mg to or from IU directly.
How many units do I draw for a 2 IU dose from a 36 IU vial in 1 mL?
5.56 units on a U-100 syringe. That reading comes from the IU dose divided by the IU vial strength, times the water volume, times 100 — the same figure whichever unit the vial and dose happen to be labelled in, since IU cancels out of this particular calculation.

References

  1. World Health Organization / NIBSC. WHO International Standard, Somatropin (Recombinant DNA-Derived Human Growth Hormone), NIBSC code 98/574, Instructions for Use, Version 6.0, dated 14/11/2025. https://nibsc.org/documents/ifu/98-574.pdf

For non-somatropin peptides, see the general peptide dosage calculator.