HGH Dosing: How to Convert IU to mg (and Back)

A real HGH vial almost always lists IU or mg, not both. Here is the standard conversion, worked examples on real vial sizes, and a reference table so you can go either direction.

I ran HGH for about two months and saw amazing results, better recovery, and it helped me hold onto muscle while I was cutting weight. The IU thing messed me up at first. I had to learn what an IU even was, then figure out how to convert it into milligrams before I could make sense of my own vial. That conversion is exactly what this post walks through.

Quick answer

IU = mg × 3, and mg = IU ÷ 3. That 1 mg = 3 IU ratio is the standard labeling convention for somatropin (HGH) and what Stackeddd's calculator uses. A vial typically lists only one of the two units. Converting is how you get the number you need for reconstitution or dosing math.

IU and mg are two different ways to measure the same thing

mg (milligrams) is a unit of mass (how much the hormone weighs). IU (International Units) is a unit of biological activity, a standardized measure meant to represent a consistent effect regardless of small differences in exact purity between batches. They are two different ways of describing the same substance, not two different substances, which is exactly why a fixed ratio connects them.

Vial labels usually pick one unit and stick to it. Some vials are labeled “10 IU,” others “12 mg,” rarely both on the same label. Your job, when the number you need is in the other unit, is converting, not guessing.

The conversion: 1 mg = 3 IU

The conversion for somatropin (HGH) is 1 mg = 3 IU. That gives two directions:

IU = mg × 3 and mg = IU ÷ 3

This is not merely a labeling habit, which is worth knowing, because plenty of places present it as one. It comes from the WHO 2nd International Standard for Somatropin, code 98/574, established by the WHO Expert Committee on Biological Standardization in 2001, whose assigned specific activity is 3.0 IU/mg. Approved somatropin products are calibrated against that standard, which is why the number is consistent across manufacturers rather than a convention each one picked.

You will still see figures cited around 2.7 IU/mg, and now the reason is clear rather than mysterious: that traces to earlier reference material, including the standard 98/574 replaced and older pituitary-derived growth hormone, which was not the same preparation and did not carry the same assigned activity. For recombinant somatropin, 3.0 IU/mg is the internationally agreed figure. If a specific product's own label states something different, defer to that label.

Reading real vial labels

Applying the conversion to vial sizes you'll see on a label:

A 10 IU vial ≈ 3.33 mg
A 12 mg vial ≈ 36 IU
A 36 IU vial ≈ 12 mg

Vial label	mg	IU
4 mg	4	12
5.30 mg	5.30	15.90
10 mg	10	30
12 mg	12	36
15 mg	15	45
10 IU	3.33	10
12 IU	4	12
15 IU	5	15
20 IU	6.67	20
30 IU	10	30
36 IU	12	36

Bold = the unit printed on that vial; the other column is the converted value.

IU = mg × 3

mg to IU is a fixed proportional relationship, not a curve. Every extra mg adds exactly 3 IU.

Once you know the mg or IU total, reconstitution is separate math

Converting the vial's total is only step one. How much of that ends up in each injection depends on how much bacteriostatic water you add and what dose you draw. The same reconstitution math is covered in Peptide Reconstitution Math. Dosing in syringe units after reconstitution follows the same units-per-mL logic as any other reconstituted compound.

What I was worried about

My biggest fear going in was acromegaly and insulin resistance, the two things that come up in nearly every conversation about HGH. GH’s effect on blood sugar is dose-dependent. The insulin resistance and acromegaly features come from chronic, supraphysiologic GH; lower exposure carries lower risk, but lower is not the same as safe (GH raises blood sugar at any meaningful dose), and clinical GH is titrated to IGF-1 blood levels under medical supervision, not to a fixed IU number.

That distinction, an IU on a vial label versus an IGF-1 number in blood work, is part of why getting the conversion itself exactly right matters. I built the mg ↔ IU converter into the HGH tab of the Dosing Calculator for that reason, so you can run the conversion, the reconstitution, and the per-injection syringe math together, for free, using the same 1 mg = 3 IU convention as this post and the Stackeddd app.

Frequently asked questions

What is the actual difference between IU and mg for HGH?

mg (milligrams) is a measure of mass: how much the hormone weighs. IU (International Units) is a measure of biological activity, standardized so a given number of units produces a consistent effect regardless of exact molecular purity. They describe the same substance from two different angles, which is exactly why a fixed conversion factor between them exists.

Why do some sources cite a different ratio than 1 mg = 3 IU?

You will see figures in the roughly 2.7-3 IU/mg range across different sources, reflecting small differences in how potency was originally standardized. 1 mg = 3 IU is the common labeling convention in practice and is what Stackeddd's calculator uses. If a specific product's label states a different ratio, use the number on that label.

If my vial is labeled "10 IU," how many mg is that?

IU ÷ 3 = mg, so 10 IU ÷ 3 ≈ 3.33 mg. A real vial almost always lists one unit or the other, not both. Converting is how you get the number in whichever unit you need.

If my vial is labeled "12 mg," how many IU is that?

mg × 3 = IU, so 12 mg × 3 = 36 IU. Same relationship, opposite direction.

Can Stackeddd do this conversion for me?

Yes. The HGH tab of the Dosing Calculator includes a live mg ↔ IU converter alongside the reconstitution and per-injection math, so you can enter whichever unit is on your vial and get the other automatically.

Sources

WHO 2nd International Standard for Somatropin (recombinant DNA-derived human growth hormone), NIBSC code 98/574 — the reference preparation carrying the assigned specific activity of 3.0 IU/mg. NIBSC.
Bristow & Jeffcoate et al., The Second International Standard for Somatropin: Preparation and Calibration in an International Collaborative Study — the calibration behind that figure, and the replacement of the earlier standard 88/624. Biologicals.

This is educational information, not medical advice. It covers unit conversion only: your dose is set by your prescribing physician.