Why there's no single "mcg to IU" conversion
IU (International Unit) measures biological activity rather than mass. Unlike a conversion such as grams to ounces, there is no single fixed number that converts micrograms to IU. The conversion factor depends on the specific substance and, for some vitamins, its chemical form. The NIH Office of Dietary Supplements explains that IU is a substance-specific measure of biological activity.
This converter focuses on vitamins A, D, and E because these are the vitamin IU-to-mass conversions supported by the reference sources used here. Other substances may also have International Unit definitions, but their conversions are substance-specific and aren't included in this tool.
Vitamin A: mcg to IU
Vitamin A is the trickiest of the three, because "how much vitamin A" depends on where it came from. Preformed vitamin A (retinol) and beta-carotene from a supplement convert the same way, but beta-carotene and related carotenoids from food convert very differently — food sources are absorbed and used far less efficiently, so it takes a lot more of them to equal the same amount of active vitamin A.
This is exactly why current labels use mcg RAE (Retinol Activity Equivalents) rather than plain mcg for vitamin A — and why this converter labels its vitamin A input the same way. The NIH ODS vitamin A fact sheet lays out a striking example: 900 mcg RAE of vitamin A works out to 3,000 IU as retinol, 3,000 IU as supplemental beta-carotene, 18,000 IU as dietary beta-carotene, or 36,000 IU as dietary alpha-carotene or beta-cryptoxanthin — a 12× difference for the exact same 900 mcg RAE, depending entirely on the source.
Vitamin D: mcg to IU
Vitamin D is the simplest of the three: 1 mcg = 40 IU, and this holds for both D2 (ergocalciferol) and D3 (cholecalciferol) — there's no separate factor to choose between them, which is why this converter doesn't ask you to pick a form for vitamin D at all.
Vitamin E: mg to IU
Vitamin E is measured in milligrams rather than micrograms, and like vitamin A, the conversion depends on the chemical form. The factors differ between natural and synthetic vitamin E and also between the free tocopherol and acetate ester forms. For example, 1 IU corresponds to 0.67 mg of natural RRR-alpha-tocopherol but 0.45 mg of synthetic all-rac-alpha-tocopherol. The free-form factors come from FDA guidance; the acetate ester factors are documented by USP and EFSA.
If you need to convert mass units prior to calculating IU? Use our mg to mcg converter and mcg to mg converter.
How to convert mcg to IU
Pick the vitamin, and where relevant, the specific form. Divide the amount in mcg (or mg) by that form's mcg-per-IU (or mg-per-IU) figure to get IU — or multiply IU by that same figure to go the other way. This converter does that math automatically in both directions as you type.
IU = mass ÷ mass per IU
To convert IU back to mcg or mg, multiply the IU amount by the same mass-per-IU factor:
Mass = IU × mass per IU
For example, vitamin D uses 0.025 mcg per IU, so 25 mcg of vitamin D equals 1,000 IU.
Frequently asked questions
Why does vitamin A need a "form" selector but vitamin D doesn't?
Because the body converts different sources of vitamin A into active retinol at very different efficiencies, so the IU figure genuinely depends on the source. Vitamin D doesn't have this issue — D2 and D3 use the same conversion factor, so there's nothing to choose between.
What if I don't know which form of vitamin A or E I have?
Check the supplement label or product packaging. It may list retinol or a retinyl ester for preformed vitamin A, or beta-carotene and other provitamin A carotenoids. For foods, the source matters: animal foods such as liver, fish, eggs, and dairy can provide preformed vitamin A, while many colorful fruits and vegetables provide provitamin A carotenoids.
References & Sources
- National Institutes of Health, Office of Dietary Supplements & USDA. Unit Conversions for Nutrients. dsid.od.nih.gov
- National Institutes of Health, Office of Dietary Supplements. Vitamin A — Health Professional Fact Sheet. ods.od.nih.gov
- National Institutes of Health, Office of Dietary Supplements. Vitamin D — Health Professional Fact Sheet. ods.od.nih.gov
- U.S. Food and Drug Administration. Guidance for Industry: Converting Units of Measure for Folate, Niacin, and Vitamins D and E on the Nutrition and Supplement Facts Labels. fda.gov
- European Food Safety Authority (EFSA). Scientific opinion on the tolerable upper intake level for vitamin E. EFSA Journal