mcg to IU Converter — Micrograms to International Units
Quickly convert a dose in micrograms (mcg) to International Units (IU) for vitamins A, D and E. Select the vitamin form — the conversion factor differs between natural and synthetic vitamin E, and between retinol and beta-carotene for vitamin A.
Vitamin / substance
mcg
Equivalent IU for the selected vitamin
- 1
Conversion factor
1 IU = 0.025 mcg = 40IU per mcg — divide by this factor to convert mcg into IU. - 2
IU = mcg ÷ mcg per IU
400 ÷ 0.025 = 16,000
How does this calculator work?
Convert mcg to IU by dividing by the substance-specific factor: vitamin D uses 0.025 mcg/IU (400 IU = 10 mcg), vitamin A retinol uses 0.3 mcg/IU, beta-carotene 0.6 mcg/IU, natural vitamin E 0.6706 mcg/IU, and synthetic vitamin E 0.4536 mcg/IU (2020 FDA values).
Formula
How this is calculated
The International Unit is a biological-activity measure, not a fixed mass. Each substance has its own mcg-per-IU factor established by reference standards: for vitamin D (D2 or D3), 0.025 mcg = 1 IU (so 400 IU = 10 mcg); for vitamin A as retinol, 0.3 mcg = 1 IU; for beta-carotene (provitamin A), 0.6 mcg = 1 IU; for natural vitamin E (d-α-tocopherol), 0.6706 mcg = 1 IU; and for synthetic vitamin E (dl-α-tocopherol), 0.4536 mcg = 1 IU. These are the 2020 U.S. FDA reference values used on supplement labels.
To convert, divide the mcg amount by the appropriate factor. The formula is therefore IU = mcg ÷ (mcg per IU). Conversely, mcg = IU × (mcg per IU). This calculator displays both directions so you can confirm a supplement label, translate a prescription dose, or compare products measured in different units.
Important limitations: IU values are substance-specific and differ even between forms of the same vitamin (e.g. natural vs. synthetic E). This tool covers only the four most commonly listed vitamins. For other biologics (insulin, heparin, etc.) the IU definition is set by international pharmacopoeias and differs completely — do not apply these factors to other substances.
Frequently asked questions
Vitamin D has a conversion factor of 0.025 mcg per IU (i.e. 1 IU = 0.025 mcg), which means 400 IU × 0.025 = 10 mcg. Many modern labels now show both figures since the 2020 FDA labelling update.
Yes. Natural vitamin E (d-α-tocopherol) has higher biological activity per milligram than the synthetic dl-α form — 0.6706 mcg per IU vs 0.4536 mcg per IU respectively — so the same mcg quantity translates to fewer IU for the natural form. Always check your product label.
No. Vitamins C and K are dosed only in milligrams or micrograms — they do not have IU definitions. This converter covers only A, D and E, which are the three fat-soluble vitamins that historically used IU on supplement labels.
Also known as
TG we-Calculate Editorial Team. (2026). mcg to IU Converter — Micrograms to International Units [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/mcg-to-iu-converter
TG we-Calculate Editorial Team. "mcg to IU Converter — Micrograms to International Units." TG we-Calculate. 2026. https://we-calculate.com/calculator/mcg-to-iu-converter.
TG we-Calculate Editorial Team, "mcg to IU Converter — Micrograms to International Units," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/mcg-to-iu-converter
@misc{wecalculate_mcg_to_iu_converter, title = {mcg to IU Converter — Micrograms to International Units}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/mcg-to-iu-converter}}, year = {2026}, note = {TG we-Calculate} }
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