Vitamin IU converter

Why IU needs a different factor for every vitamin

The international unit (IU) is not a weight. It was defined for each vitamin by biological effect, so 1 IU of vitamin D weighs a very different amount from 1 IU of vitamin A or E. A label that says "5,000 IU" cannot be turned into micrograms without knowing which vitamin and which chemical form it is. Modern US labels use weights instead: micrograms of retinol activity equivalents (mcg RAE) for vitamin A, micrograms for vitamin D, milligrams of alpha-tocopherol for vitamin E and micrograms of dietary folate equivalents (mcg DFE) for folate. That change is why people ask for a converter today.

How many IU are in a microgram of vitamin D?

1 mcg of vitamin D is 40 IU, so 1 IU is 0.025 mcg. The Daily Value of 20 mcg is therefore 800 IU. (National Institutes of Health, Office of Dietary Supplements).

Why is there no single IU-to-mcg factor for vitamin A?

An international unit measures biological activity, not weight, so the factor depends on the compound. One IU of retinol is 0.3 mcg of retinol activity equivalents (RAE), the same as one IU of beta-carotene in a supplement, but one IU of beta-carotene in food is only 0.05 mcg RAE and one IU of alpha-carotene or beta-cryptoxanthin from food only 0.025 mcg RAE. That is why this converter asks you for the form.

How do I convert vitamin E from IU to mg?

The NIH and FDA factors are: 1 IU of natural vitamin E (RRR, d-alpha-tocopherol) is 0.67 mg of alpha-tocopherol, and 1 IU of synthetic vitamin E (all-rac, dl-alpha-tocopherol) is 0.45 mg. In the other direction, 1 mg of alpha-tocopherol is 1.49 IU of the natural form or 2.22 IU of the synthetic form. Check the ingredient list: natural forms are written d-alpha, synthetic forms dl-alpha.

What is a dietary folate equivalent (DFE)?

The DFE corrects for the fact that folic acid is absorbed better than the folate in food. 1 mcg DFE equals 1 mcg of food folate, 0.6 mcg of folic acid from fortified food or a supplement taken with food, or 0.5 mcg of folic acid from a supplement taken on an empty stomach. So 400 mcg of folic acid taken with food counts as 667 mcg DFE.

When did labels change from IU to mcg and mg?

The FDA required the new Nutrition Facts and Supplement Facts labels starting in January 2020; companies with annual food sales under $10 million could use the old labels until January 2021. Old products and old bottles at home may still show IU, which is why people need a conversion. Vitamin D may still be shown in IU in parentheses after the mcg amount.

Is a higher amount always better?

No. Vitamin D has an adult upper intake level of 100 mcg (4,000 IU) a day, set by the National Academies, and high doses of other fat-soluble vitamins carry risks too. This converter changes units; it does not say how much you need. Ask your doctor or pharmacist about any supplement, especially if you are pregnant or take medicine.

How the conversion works

Each form has one factor, taken from the NIH Office of Dietary Supplements and the FDA: label amount = old amount × factor, and the other way round, old amount = label amount ÷ factor. The factors are 0.3 (retinol and supplement beta-carotene), 0.05 and 0.025 (carotenoids from food) for vitamin A in mcg RAE per IU; 0.025 mcg per IU for vitamin D; 0.67 and 0.45 mg per IU for natural and synthetic vitamin E; and, for folate, 1 mcg DFE per 1 mcg of food folate, 1 ÷ 0.6 per mcg of folic acid taken with food and 1 ÷ 0.5 per mcg taken on an empty stomach. The percent of Daily Value uses the label values in 21 CFR 101.9: 900 mcg RAE for vitamin A, 20 mcg for vitamin D, 15 mg for vitamin E and 400 mcg DFE for folate.

Worked examples

Vitamin A. An older multivitamin lists 5,000 IU of vitamin A as retinol. That is 5,000 × 0.3 = 1,500 mcg RAE, or 167% of the 900 mcg RAE Daily Value. The NIH fact sheet shows how much the source matters: the adult male RDA of 900 mcg RAE equals 3,000 IU of retinol or supplement beta-carotene, but 18,000 IU of beta-carotene from food and 36,000 IU of alpha-carotene or beta-cryptoxanthin from food.

Vitamin D. A bottle says 800 IU. That is 20 mcg, exactly the Daily Value of 20 mcg (100%). A 4,000 IU capsule is 100 mcg, which equals the adult upper intake level of 100 mcg.

Vitamin E. A softgel labeled 400 IU natural vitamin E contains 400 × 0.67 = 268 mg of alpha-tocopherol; the same 400 IU as synthetic vitamin E contains 180 mg. Going the other way, the 15 mg Daily Value is 22.4 IU of the natural form or 33.3 IU of the synthetic form, the figures the NIH uses in its own example.

Folate. A supplement with 400 mcg of folic acid taken with a meal counts as 667 mcg DFE; taken on an empty stomach it counts as 800 mcg DFE, because folic acid is absorbed better than the folate in food (at least 85 percent against about 50 percent, according to the NIH).

Common mistakes

• Using one factor for all vitamin A. A "0.3" taken from a retinol label applied to food beta-carotene overstates the vitamin A activity six-fold, because the food factor is 0.05.
• Mixing natural and synthetic vitamin E. Synthetic alpha-tocopherol is about half as active as the natural form (NIH). On the label, 1 mg of alpha-tocopherol equals 1 mg of RRR-alpha-tocopherol or 2 mg of all-rac-alpha-tocopherol (21 CFR 101.9, note 4 to the table). Some conversion tables show 0.9 mg per IU for synthetic vitamin E; we did not find that figure in the NIH or eCFR text we read, and we use the NIH value of 0.45 mg of alpha-tocopherol per IU.
• Treating the folic acid milligrams as DFE. A 400 mcg folic acid tablet is not 400 mcg DFE. Fortified foods and supplements are credited at 1 ÷ 0.6 mcg DFE per mcg of folic acid.
• Reading mcg RAE as retinol only. A vitamin A amount in mcg RAE may include provitamin A carotenoids; the label can voluntarily state what percent is present as beta-carotene (21 CFR 101.9(c)(8)(vi)). Two products with the same mcg RAE are not necessarily the same mix.

Sources and related calculators

Sources, read October 28, 2026: NIH Office of Dietary Supplements, Health Professional fact sheets for Vitamin A, Vitamin D, Vitamin E and Folate (conversion factors, DFE definition, label change dates, upper level for vitamin D); US Code of Federal Regulations, 21 CFR 101.9(c)(8)(iv) (Daily Values and the notes on RAE, alpha-tocopherol and DFE). This page is not endorsed by or affiliated with the NIH or the FDA. Related: calorie calculator, sodium intake calculator, added sugar calculator and fiber intake calculator.

For general information only — not medical advice. Talk with your doctor or another health professional about your own health. How we check our calculators.