Quick answer
IU stands for international unit, a measure of a vitamin's biological activity rather than its weight. US labels now give vitamins A, D and E by weight, but vitamin D may still show IU in parentheses: 1 mcg of vitamin D equals 40 IU, so 25 mcg reads 25 mcg (1,000 IU).1,2,3
Key facts
- Since the 2016 label rule, vitamin A is declared in mcg RAE, vitamin D in mcg and vitamin E in mg on Nutrition and Supplement Facts labels.1
- Vitamin D is the only one of the three that may still show IU, and only in parentheses after the mcg amount.2,4
- Larger companies had to use the new labels from January 2020, and those with under $10 million in annual sales from January 2021, so older bottles may still list IU.5
- For vitamin E, 1 IU of the natural form equals 0.67 mg and 1 IU of the synthetic form equals 0.45 mg.1,5
- For vitamin A the factor depends on the source: 1 IU of retinol or of supplemental beta-carotene is 0.3 mcg RAE, but 1 IU of beta-carotene from food is 0.05 mcg RAE.6
Why IU was used, and why it was dropped
An IU measures what a vitamin does in a biological test, not how much it weighs. FDA guidance notes that one IU of vitamin D was defined as the activity of 0.025 mcg of cholecalciferol in tests on rats and chicks, which is where 1 mcg equals 40 IU comes from.1
For vitamin A the old unit hid a real difference. FDA says IU did not reflect how much less vitamin A the body gets from carotenoids than from retinol, so the 2016 rule moved to retinol activity equivalents, which do.1
Converting IU to the units on today's labels
| Vitamin and form | 1 IU equals | Example |
|---|---|---|
| Vitamin D (D2 or D3) | 0.025 mcg | 1,000 IU = 25 mcg |
| Vitamin E, natural (d-alpha-tocopherol) | 0.67 mg | 30 IU = about 20 mg |
| Vitamin E, synthetic (dl-alpha-tocopherol) | 0.45 mg | 30 IU = 13.5 mg |
| Vitamin A as retinol | 0.3 mcg RAE | 5,000 IU = 1,500 mcg RAE |
| Vitamin A as beta-carotene in a supplement | 0.3 mcg RAE | 5,000 IU = 1,500 mcg RAE |
| Vitamin A as beta-carotene from food | 0.05 mcg RAE | 5,000 IU = 250 mcg RAE |
The old reference amounts show the size of the change. The 1993 Daily Values were 5,000 IU for vitamin A, 400 IU for vitamin D and 30 IU for vitamin E. Today's are 900 mcg RAE, 20 mcg (800 IU) and 15 mg.1,3
Reading IU on an older bottle
A vitamin A figure in IU cannot be turned into mcg RAE unless you know the source. NIH's example: the 900 mcg RAE RDA for men equals 3,000 IU if it comes from retinol or supplemental beta-carotene, but 18,000 IU if it comes from beta-carotene in food.6
For vitamin E, check whether the label says d-alpha or dl-alpha tocopherol. The synthetic dl form is only half as active by weight, which is why each of its IU converts to fewer milligrams.5
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Sources
- U.S. Food and Drug Administration. Converting Units of Measure for Folate, Niacin, and Vitamins A, D, and E on the Nutrition and Supplement Facts Labels: Guidance for Industry. August 2019. Opened 2026-10-07.
- Code of Federal Regulations, Title 21, Section 101.36: Nutrition labeling of dietary supplements. Legal Information Institute, Cornell Law School (eCFR text). Opened 2026-10-07.
- NIH Office of Dietary Supplements. Vitamin D: Fact Sheet for Health Professionals. Updated June 27, 2025. Opened 2026-10-07.
- Code of Federal Regulations, Title 21, Section 101.9: Nutrition labeling of food. Legal Information Institute, Cornell Law School (eCFR text). Opened 2026-10-07.
- NIH Office of Dietary Supplements. Vitamin E: Fact Sheet for Health Professionals. Updated March 26, 2021. Opened 2026-10-07.
- NIH Office of Dietary Supplements. Vitamin A and Carotenoids: Fact Sheet for Health Professionals. Updated March 10, 2025. Opened 2026-10-07.
This page is information, not medical advice. Talk to your doctor or pharmacist about your own situation.