International unit (IU)

An international unit (IU) measures how biologically active a substance is, not how much it weighs. The World Health Organization defines an IU for a given substance — typically a hormone, vitamin, or similar biologic — by comparing it against an international reference standard that produces a defined effect.

Why IU doesn’t convert to mg directly

Mass units like mcg and mg always relate by the same factor: 1 mg is always 1000 mcg, for anything. IU is different. Each substance measured in IU has its own potency reference set by the WHO, so the mass equivalent of “1 IU” varies from one substance to the next — there is no single, universal IU-to-mg formula. Converting IU to mg for a specific product requires that product’s own conversion factor, not a general rule.

Where IU shows up

IU labeling is common for vitamins, hormones, and other biologics where potency — not raw weight — is what matters for consistent, comparable dosing across manufacturers. It’s a separate concept from the volume marks on a U-100 syringe, which describe how much liquid is drawn, not how biologically active it is.

Because IU-to-mg conversion depends on a substance-specific factor rather than a fixed multiplier, this glossary and the mg-to-units calculator work with the mass and volume figures you enter — mg, mcg, and mL — and never invent an IU conversion where one hasn’t been supplied.