International units — IU to mg conversions for vitamins
IU measures bioactivity, not mass. Each substance has its own IU/mg factor. Confuse them and vitamin D dosing errors reach ten-fold.
The International Unit is defined per substance by biological potency, not by mass. It survives in vitamins, some hormones and clotting factors. Converting to mg or µg requires the specific factor.
The exact factors
Vitamin A: 1 IU = 0.3 µg retinol (0.6 µg β-carotene) > Vitamin D₂/D₃: 1 IU = 0.025 µg (40 IU per µg) > Vitamin E (RRR-α-tocopherol, natural): 1 IU = 0.67 mg > Vitamin E (all-rac, synthetic): 1 IU = 0.91 mg > Insulin (human): 1 IU = 34.7 µg (historical rabbit assay)
Where IU still appears
Vitamin D supplements: still often labelled in IU. Insulin: uniquely IU-only, dosing never mass-based. Heparin: activity-based, IU/mL. Some enzymes and clotting factors.
Worked example — vitamin D3
Prescription 5000 IU vitamin D₃ = 125 µg = 0.125 mg. Confuse mg with IU and prescribe 5000 mg — a lethal overdose (~40 000× toxic threshold). Documented pharmacy errors have caused hospitalisation.
Recap
IU is bioactivity per substance. Look up the specific factor; do not assume.
Try the converters mentioned in this article
FAQ
Is 1 IU insulin always the same?+
Modern human insulin: 1 IU = 34.7 µg. Older animal insulins used the same IU by historical assay.
Why does synthetic vitamin E need more mg per IU?+
Racemic mixture — only some isomers are bioactive.
This article was written by The Turbo Unit Converter engineering desk (Applied metrology & engineering units) and last reviewed on 2026-07-07 against NIST SP 811 and the BIPM SI Brochure. Read our full editorial policy.