What is an IU on a vitamin label (and why you can't convert IU to mg with one number)

An IU (international unit) tells you how strongly a dose acts in the body, not how much it weighs. So 1 IU of vitamin D is 0.025 mcg, while 1 IU of natural vitamin E is 0.67 mg, 26,800 times heavier. For a quick answer, open the IU to mcg and mg converter. This page explains why it works the way it does and how to avoid mistakes on the label.

What IU means on a label

A milligram is a weight. An IU is an effect. When the molecules are identical, you don’t notice the difference: 1 mcg of vitamin D3 always gives 40 IU. But some vitamins come in forms that act more weakly or more strongly at the same weight.

Two examples from NIH ODS:

• synthetic vitamin E (dl-alpha-tocopherol) is a mix of eight similar molecules, and your body keeps only four, so the FDA counts 2 mg of it as 1 mg of vitamin E. The mg on the label show the part that counts, which is why 1 IU of synthetic gives 0.45 mg and 1 IU of natural gives 0.67 mg;
• beta-carotene from carrots gives you less vitamin A than beta-carotene in a capsule. From food, 1 IU is worth 0.05 mcg RAE; from a supplement, 0.3 mcg.

Which vitamins are measured in IU

On supplement labels you’ll meet IU for three vitamins:

• A – retinol (retinyl palmitate, retinyl acetate) and beta-carotene;
• D – D3 (cholecalciferol) and D2 (ergocalciferol), which share one factor;
• E – natural d-alpha-tocopherol and synthetic dl-alpha, plus their esters (acetate, succinate).

B vitamins, vitamins C and K and minerals are given in mg or mcg, so there’s nothing to convert.

IU to mg: the factor for each vitamin

For beta-carotene, mcg RAE (retinol activity equivalents) isn’t the weight of the carotene itself but how much retinol your body gets from it. Vitamin A has its own page: vitamins A and E.

4,000 IU in mg

Most people asking this mean vitamin D. 4,000 IU of vitamin D is 100 mcg, or 0.1 mg, and that’s the adult upper limit set by EFSA and the NHS. The full chart from 2.5 to 250 mcg, with children’s limits, is on vitamin D mcg to IU.

For retinol, 4,000 IU is 1,200 mcg RAE (1.2 mg). For vitamin E the number gets silly: 2,680 mg of the natural form, almost nine times EFSA’s 300 mg limit. Read the label again, because it may well say 400 IU. Vitamin E conversions are covered in detail in vitamin E IU to mg.

1 IU and 10 IU in mg

Small numbers cause the most confusion:

• vitamin D: 1 IU is 0.025 mcg, and 10 IU is 0.25 mcg (0.00025 mg), forty times less than the 10 mcg the NHS advises each day;
• vitamin E: 10 IU is 6.7 mg of the natural form or 4.5 mg of the synthetic form;
• retinol: 10 IU is 3 mcg RAE.

Why labels now use mcg and mg

In the US, the new Supplement Facts labels became mandatory in January 2020, and in January 2021 for companies with annual sales under $10 million. Vitamin D now appears in mcg, and the FDA allows IU in brackets alongside. Vitamin A is in mcg RAE and vitamin E in mg. An FDA guidance document from 2019 shows manufacturers how to convert the old IU, using the same factors as the table above.

UK and EU labels give vitamins in µg and mg with a % NRV, and the NHS gives its vitamin D advice in micrograms. Older articles, forum posts and the front of many bottles still talk in IU. So a bottle from iHerb saying “25 mcg (1,000 IU)” and a friend’s advice to take “1,000 IU” mean exactly the same dose.

How to read IU on an iHerb label

1. Ignore the front of the bottle and go to the Supplement Facts panel.
2. Find the unit: mcg, mcg RAE or mg. IU in brackets is a hint, not the main figure.
3. Read the form after “as” in the brackets: cholecalciferol, retinyl palmitate, d-alpha tocopheryl succinate.
4. If you need IU, use the factor for that exact form.
5. Add up the same vitamin across every bottle you take.

Where people trip up on the label:

• seeing “400 IU” on two vitamin E bottles and assuming the doses match. With d-alpha it’s 268 mg, with dl-alpha 180 mg;
• thinking “1,000 IU” from a friend and “25 mcg” on the bottle are different doses, and buying a second bottle to be safe;
• reading % DV as a limit. It’s a share of the US daily value, and 125% for vitamin D is still well within the limit;
• treating % DV on a US bottle and % NRV on a UK or EU pack as the same thing, when the reference amounts differ.

Slips in the maths itself, such as dividing vitamin E by 40, are listed on the IU converter page.

The rest of the panel is explained in how to read a supplement label, and vitamin D doses and forms in how much vitamin D3 to take.