Mg to IU Converter

An international unit (IU) measures biological activity, not weight, so no single number turns mg into IU. Each vitamin, and sometimes each form of a vitamin, has its own factor, set out in FDA labeling guidance⁴.

What Mg to IU Conversion Measures

This tool changes how an amount is written, not the amount. 25 mcg and 1,000 IU of vitamin D are the same amount written two ways¹. It covers only vitamins D, E and A and beta-carotene, the nutrients with official factors. It does not say whether an amount is high or low.

The Formula

Change the amount to mcg, then divide by the mcg in 1 IU for the form you chose.

IU = amount in mcg / mcg per IU

Amount in mcg = IU x mcg per IU

The factors come from the FDA guidance written for the switch from IU to mcg and mg on US labels⁴.

How This Calculator Computes Your Result

Pick the substance and form first. The tool shows no result until you do. Then type the amount and choose mg or mcg. For vitamins D and A the toggle starts on mcg, as on US labels. Vitamin A takes mcg RAE (retinol activity equivalents), the number printed on the label. IU is rounded to 2 decimals below 10 IU and to 1 decimal from 10 IU up. The swap button converts IU back to mg or mcg without rounding. "Something else" gives a note instead of a result.

What Your Result Means

Your result is the same amount expressed in IU. This table shows the factor used for each option.

For the vitamin A rows, 1 mg means 1 mg RAE, which is 1,000 mcg RAE. The factors come from the NIH fact sheets for vitamin D¹, vitamin E² and vitamin A³, and from FDA for beta-carotene by weight⁴.

A quick sanity check catches a wrong toggle. Vitamin D in IU is 40 times the mcg figure. Vitamin E in IU is about 1.5 times the mg figure (natural) or 2.2 times (synthetic). Vitamin A as retinol is about 3.3 IU per mcg RAE.

Old Labels in IU, New Labels in mcg and mg

FDA updated the Nutrition Facts and Supplement Facts labels in 2016. Vitamin A is now listed in mcg RAE, vitamin D in mcg and vitamin E in mg⁴. Larger companies switched by January 2020, and companies with under $10 million in sales by January 2021². Vitamin D labels may still show IU in parentheses¹.

The Daily Values changed too, so the % Daily Value can change even when the amount in the product does not.

Old values are from the 2015 edition of the federal labeling rule⁵ and new ones from the NIH fact sheets¹ ² ³. A product with 25 mcg (1,000 IU) of vitamin D showed 250% of the Daily Value on an old label and 125% on a new one. To compare labels, convert both amounts to one unit and compare amounts, not percentages.

Why Vitamin A Needs the Source

RAE reflects how well the body turns each source into vitamin A. One mcg RAE equals 1 mcg of retinol, 2 mcg of beta-carotene from supplements, 12 mcg of beta-carotene from food, or 24 mcg of alpha-carotene or beta-cryptoxanthin³. So 900 mcg RAE is anywhere from 3,000 IU (retinol) to 36,000 IU (alpha-carotene or beta-cryptoxanthin)³.

RAE can only be converted into IU when the sources are known³. FDA applies each factor only to products that are not a mixture, and the supplement beta-carotene factor only to purified beta-carotene in oil⁴. If a label says vitamin A "as retinyl palmitate and beta-carotene" with no split, no single IU figure is correct.

Natural Versus Synthetic Vitamin E

Natural vitamin E is usually labeled d-alpha-tocopherol and synthetic vitamin E dl-alpha-tocopherol². The synthetic form is about half as active, and each form has its own IU factor². FDA uses the same factors for ester forms, such as d-alpha-tocopheryl acetate⁴.

The mg on a current label counts active vitamin E. FDA counts 2 mg of synthetic vitamin E as 1 mg on the label⁴, so the synthetic ingredient weighs about twice the label figure. For vitamin E, the result also shows the IU for the other form, which helps when a label does not say d or dl.

Insulin, Heparin and Other Medicines

Insulin, heparin, many vaccines and some hormones are measured in units. For many biological medicines, activity cannot be expressed as a mass, so it is measured with lab tests against reference materials of known strength⁶. Heparin shows why. In October 2009 the US heparin unit was aligned with the international standard unit, and heparin made after that was about 10% less potent, though still labeled in units⁷.

A unit of a medicine is not a fixed weight, so this tool does not convert medicines. Medicine doses must follow the prescription and the product label. ISMP also advises writing "units" in full, because "IU" can be misread as IV (intravenous) or the number 10⁸.