HGH Calculator — Somatropin & Norditropin Dosage, mg to IU
Recombinant human growth hormone. The one peptide on this site with an official milligram-to-IU conversion — 3 IU per mg.
Written by Kashif Khan, who builds and maintains this site and is not a clinician. Every figure links to its primary source.
FDA-approved
Several approved somatropin products exist, most supplied as lyophilised powder. Genotropin uses a two-chamber cartridge where the device performs the reconstitution. Dosing on current US labels is in milligrams, often mg/kg per week for paediatric indications.
Key facts
- Molecular weight
- 22,125 Da191 amino acids, recombinant.
- Half-life
- Not published. No accessible human pharmacokinetic study reports one, so any specific figure quoted elsewhere has no primary source behind it.
- Also sold as
- growth hormone, Genotropin, Humatrope, Norditropin, Omnitrope
How good is the evidence?
Well established and approved, with the important caveat that most somatropin prescribing is for specific deficiency indications rather than the uses it is sought for online.
Norditropin FlexPro: nothing to reconstitute, and no syringe units
Norditropin is the somatropin most people search a calculator for, and it is the one that needs the least arithmetic. It is supplied as a ready-made clear solution in a prefilled FlexPro pen — there is no powder, no diluent and no mixing step. The label lists four pens: 5 mg/1.5 mL (orange), 10 mg/1.5 mL (blue), 15 mg/1.5 mL (green) and 30 mg/3 mL (purple).
Those strengths work out to about 3.3, 6.7, 10 and 10 mg/mL. The pen is dialled in milligrams, so the dose you select is the dose delivered; syringe units never enter into it. The calculator on this page only matters if you are drawing Norditropin into a separate syringe, which the pen is not designed for — and in that case the concentration above is the number to enter, not one you chose.
In IU, multiply by 3 using the WHO factor: the pens hold 15, 30, 45 and 90 IU. The Norditropin label itself doses in mg/kg per week and does not print an mg-to-IU conversion. After the first injection a pen keeps for 4 weeks refrigerated at 2°C to 8°C, or 3 weeks at up to 25°C (77°F).
The one official mg-to-IU conversion, and where it stops
Somatropin is the only compound on this site with a genuine, official mass-to-international-unit conversion: 3.0 IU per milligram, set by the WHO International Standard for somatropin. Everywhere else on this site, an IU figure quoted for a peptide has no such basis — international units are defined per substance by a reference preparation, and most compounds sold as peptides have none.
The conversion is also the single most common source of confusion on this page, because it does not interact with the syringe at all. IU and milligrams both measure the drug; syringe units measure liquid. A 12 mg cartridge holds 36 IU regardless of how it is reconstituted, and the number of syringe units that carries 1 mg depends entirely on the concentration: 24 units in a Humatrope 12 mg cartridge at its labelled 4.17 mg/mL, but about 8 units in a Genotropin 12 mg cartridge at 12 mg/mL. Three different quantities, and only one of them is printed on the barrel.
Be careful with the labelled concentration in particular. Humatrope's label states 2.08, 4.17 and 8.33 mg/mL for the 6, 12 and 24 mg cartridges — figures that are not what you get by dividing by the nominal 3 mL, because the labelled concentration accounts for the actual delivered diluent volume. The label's number governs, not your own division.
How long a reconstituted cartridge lasts
Somatropin is one of the few peptides where the in-use window after reconstitution is published, and it varies by product rather than by molecule. Reconstituted Humatrope cartridges are stable for up to 28 days refrigerated at 2°C to 8°C (36°F to 46°F), must not be left at room temperature for more than 30 minutes a day, and must not be frozen.
Genotropin splits even within its own range: the 5 mg and 12 mg cartridges, whose diluent contains a preservative, may be stored refrigerated for 28 days after reconstitution, while the 5.8 mg and 13.8 mg cartridges are given 21 days. The preservative-free MINIQUICK is a different case again and is not a multi-day container at all.
Three windows, one hormone. The variable is the formulation and its diluent, not the peptide — which is exactly why a general rule about reconstituted peptides cannot be derived from any of them.
Common questions
Sources
Every number on this page comes from one of the documents below. Where no reliable source exists, the page says so instead of filling the gap.
- FDA labelFDA / DailyMedHUMATROPE (somatropin) for injection — FDA prescribing information
- FDA labelFDA / DailyMedGENOTROPIN (somatropin) for injection — FDA prescribing information
- FDA labelFDA / DailyMed· 2025-07NORDITROPIN (somatropin) injection — FDA prescribing information
- ReferenceWHO / NIBSC· 2001WHO 2nd International Standard for Somatropin (NIBSC code 98/574) — 3.0 IU per mg
- GuidelineInternational Organization for Standardization· 2016-03ISO 8537:2016 — Sterile single-use syringes, with or without needle, for insulin (scope and requirements; full text not public)