IU to mg for Peptides: Why There's No Universal Factor

Sooner or later a peptide or a forum thread hands you a dose in IU (International Units) and you want it in milligrams so you can run the reconstitution math. The natural instinct is to search for “the IU to mg conversion” as if it were like inches to centimeters. It isn’t. There is no universal IU-to-mg factor, and understanding why is the difference between a correct dose and a dangerous guess.

This piece is mostly conceptual rather than arithmetic, because the trap here is assuming a conversion exists when it doesn’t. If your product is labeled in mass units, the peptide dose and unit converter handles the mcg, mg, mL, and syringe-unit math directly. But first you have to know when a clean conversion is even possible.

What an IU measures

A milligram is a unit of mass: a physical count of how much substance is present. An International Unit is completely different: it measures biological activity or potency, defined against an official reference preparation. The whole point of the IU system is that two batches of a biological product can have the same effect even if their exact mass differs slightly due to purity, folding, or formulation. So the IU standardizes the action, not the weight.

Because activity-per-milligram varies from substance to substance, the number of IU in one milligram is different for every compound. Insulin, HGH, and various hormones each have their own IU definition, fixed by international standards committees. There is no arithmetic that spans them all: the relationship between activity and mass is a biological property of each molecule, not a general conversion.

The rule: An IU-to-mg factor is always substance-specific. You must find the factor for your exact peptide, and never carry a number over from a different one.

This is also why the general guide to peptide dosing units: mcg, mg, mL, and IU treats IU as the one unit that doesn’t play by simple multiplication rules.

The HGH example: 3 IU per mg

The most commonly cited example is recombinant human growth hormone (somatropin). The World Health Organization’s international standard for somatropin defines its specific activity as 3.0 IU per mg. So for HGH, and HGH alone:

HGH	Equivalent
1 mg	3 IU
1 IU	~0.333 mg (333 mcg)
2 IU	~0.667 mg
3 IU	1 mg
10 IU	~3.33 mg

So if a somatropin protocol says “2 IU,” that’s roughly 0.667 mg (667 mcg) of hormone, which you can then run through concentration math to get a draw volume. Historically, older HGH preparations used slightly different factors (figures around 2.6-2.7 IU/mg appear in earlier literature), which is exactly why you should confirm the standard for the specific product rather than trusting a round number from memory.

Critically: this 3 IU/mg factor applies to HGH and nothing else. Do not apply it to a growth-hormone-releasing peptide, a research peptide, or anything else labeled in IU. Those either have their own official factor or (for most research peptides) no IU standard at all, in which case a dose written in “IU” is meaningless and should be treated with suspicion.

The insulin-unit confusion

There’s a second, sneakier source of error. The “units” printed on an insulin syringe are not IU of biological activity — they’re a volume marking. On a standard U-100 syringe, 100 “units” equals 1 mL of liquid, so one syringe unit is 0.01 mL regardless of what’s dissolved in it. That’s a volume, full stop.

Meanwhile, an “IU” of a hormone like HGH is a measure of potency. The two happen to share the word “unit,” which causes real mix-ups. When someone doses HGH, they convert IU → mg → mL, then read the mL on the syringe’s volume scale. The syringe’s “units” and the hormone’s “IU” are two different concepts that only meet after you’ve done the concentration math. Keep them mentally separated:

Term	What it measures	Fixed relationship?
IU (International Unit)	Biological activity/potency	Substance-specific (e.g., HGH = 3 IU/mg)
Insulin-syringe “unit”	Volume of liquid	Yes: 100 units = 1 mL (U-100)
mg / mcg	Mass of substance	Yes: 1 mg = 1000 mcg

How to convert IU to mg, when a factor exists

If, and only if, you have a verified IU-to-mg factor for your specific substance, the process is straightforward:

1. Convert IU to mg using the substance’s own factor. For HGH: mg = IU ÷ 3.
2. Convert mg to mcg if your dosing is in mcg: multiply by 1000.
3. Find your concentration (mg/mL) from the reconstitution; plan this in the peptide reconstitution calculator.
4. Convert to a draw volume: mL = dose ÷ concentration, then units = mL × 100.

Worked example for HGH: a 4 IU dose → 4 ÷ 3 = 1.33 mg → 1333 mcg. If the vial is 10 mg in 2 mL (5 mg/mL = 5000 mcg/mL), then 1333 ÷ 5000 = 0.267 mL = about 27 units. Every step after step 1 is the same deterministic math used for any mass-based peptide dose.

Why this matters for dosing frequency

IU-labeled compounds like HGH are also dosed on their own schedules, driven by how long they stay active in the body, a separate question from unit conversion. If you’re weighing how often to dose, the explainer on peptide half-life and why dosing frequency varies covers how activity over time, not just amount per shot, shapes a protocol.

The takeaway

• There is no universal IU-to-mg conversion. The factor is a biological property of each substance.
• HGH is ~3 IU/mg per the WHO standard; use it only for HGH.
• A research peptide labeled in “IU” with no recognized standard is a red flag, not a puzzle to solve with someone else’s factor.
• Insulin-syringe “units” are volume, not IU. Don’t conflate them.

Most research peptides are not approved for human use and are sold for laboratory research only; potency and identity are frequently unverified, and an “IU” on such a label may not correspond to any real standard. This is educational, not medical advice. Consult a qualified clinician before acting on any protocol. Once you’ve established the correct factor and converted to mass, the peptide dose and unit converter will carry you the rest of the way from mg to mL to syringe units.

Sources

• HGH specific activity of 3.0 IU/mg: WHO Second International Standard for somatropin (see reference below).
• Insulin-syringe “unit” as a volume marking (100 units = 1 mL on U-100): standard insulin syringe calibration convention.
• All mg/mcg/mL/unit conversions shown are deterministic arithmetic once a substance-specific IU-to-mg factor is known.