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IU vs mg vs mcg: The Unit Mix-Up That Fuels Half the Confusion in Peptide Research
Three little abbreviations cause an outsized share of confusion in peptide discussions: mg, mcg and IU. They look similar, they get typed fast, and they are not the same kind of thing. Here is the plain guide.
Key takeaways
- 1 mg equals 1,000 mcg (micrograms). That conversion never changes.
- An international unit (IU) measures biological activity, not mass.
- The IU-to-mass conversion is different for every substance.
- Most research peptides are labeled in mg, so IU rarely applies.
- Always write the unit, never just a number.
Mass units: mg and mcg
A milligram is a thousandth of a gram. A microgram is a thousandth of a milligram. A 5 mg vial holds 5,000 mcg. A slip of a factor of a thousand is the most common arithmetic error in this area, so spell out the unit each time. The same care appears in our concentration post.
What an IU is
An international unit is defined by what a substance does in a standard test, not by weight. For hormones such as insulin and growth hormone, a reference preparation sets the unit. That is why 1 IU of one hormone has a completely different mass than 1 IU of another, and why a single conversion factor cannot exist. The history of standardizing insulin is in the Collip story.
Why it matters here
- A label that says only a number, such as 5, is incomplete.
- Do not apply a conversion factor from one hormone to another.
- When a product is described in mg, treat all math in mass units.
- Net peptide content can make the true mass of peptide lower than the powder weight, as in our salt forms post.
A habit that prevents mistakes
Write calculations out in full, including units, and cancel them the way you did in school. If the units do not cancel to what you expect, the math is wrong. Documentation of what is in each lot is on our lab reports page.
Related reading
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For laboratory research use only. Not for human or veterinary use, and not intended to diagnose, treat, cure or prevent any condition. This article is educational and is not medical advice.