Why IU, mcg and mg are not interchangeable
An International Unit (IU) measures biological activity, not mass — so the same IU amount converts to a different mass depending on the vitamin and its chemical form. The FDA's 2016 Nutrition and Supplement Facts label rules require mcg or mg instead of IU for vitamins D, A and E, using the conversion factors below.
Vitamin D: 1 mcg = 40 IU, so mcg = IU × 0.025 (same for D2 and D3). Vitamin A: mcg RAE = IU × 0.30 for preformed vitamin A or supplemental beta-carotene, IU × 0.05 for dietary beta-carotene, and IU × 0.025 for other food provitamin A carotenoids. Vitamin E: mg alpha-tocopherol = IU × 0.67 for the natural form or IU × 0.45 for the synthetic form.
The %DV above uses the FDA's 2016 Reference Daily Intake: 20 mcg for vitamin D, 900 mcg RAE for vitamin A, and 15 mg alpha-tocopherol for vitamin E.
How do I convert IU to mcg for vitamin D, A or E?
Multiply the IU amount by the FDA's 2016 label factor for that vitamin and form: 0.025 for vitamin D, 0.30/0.05/0.025 for vitamin A depending on source, and 0.67 or 0.45 for natural or synthetic vitamin E. For example, 1,000 IU of vitamin D3 equals 1,000 × 0.025 = 25 mcg.
Steps to convert
- Identify the vitamin (D, A, or E) printed on the supplement label.
- Select the exact form listed in the ingredients — for vitamin A this changes the factor from 0.025 up to 0.30; for vitamin E, natural (0.67) versus synthetic (0.45) changes the result by nearly 50%.
- Choose the conversion direction: IU to mcg/mg to read a label in the newer format, or mcg/mg to IU to compare with an older bottle.
- Enter the number from the label. The tool multiplies (or divides) by the FDA factor and shows both the converted amount and %DV against the 2016 RDI.
FDA 2016 conversion formulas
Vitamin D: mcg = IU x 0.025 | Vitamin A: mcg RAE = IU x (0.30, 0.30, 0.05, or 0.025 depending on form) | Vitamin E: mg alpha-tocopherol = IU x (0.67 natural or 0.45 synthetic)
- IU = International Unit, a measure of biological activity defined per vitamin, not a fixed mass
- 0.025 = the single factor FDA assigns to both vitamin D2 and D3 (1 mcg = 40 IU)
- 0.30 = factor for preformed vitamin A (retinol) and supplemental beta-carotene
- 0.05 = factor for dietary (food-source) beta-carotene
- 0.025 = factor for other food provitamin A carotenoids such as alpha-carotene and beta-cryptoxanthin
- 0.67 / 0.45 = factors for natural (RRR-) versus synthetic (all-rac-) alpha-tocopherol
Example conversions for common supplement doses
| Supplement dose | Vitamin & form | Converted amount | %DV (2016 RDI) |
|---|
| 1,000 IU | Vitamin D (D2/D3, x0.025) | 25 mcg | 125% |
| 2,000 IU | Vitamin D (D2/D3, x0.025) | 50 mcg | 250% |
| 5,000 IU | Vitamin D (D2/D3, x0.025) | 125 mcg | 625% |
| 400 IU | Vitamin E, natural (x0.67) | 268 mg | 1,786.7% |
| 400 IU | Vitamin E, synthetic (x0.45) | 180 mg | 1,200% |
| 5,000 IU | Vitamin A, preformed/retinol (x0.30) | 1,500 mcg RAE | 166.7% |
Frequently asked questions
Why did the FDA change vitamin D, A and E labels from IU to mcg or mg?
It follows the FDA's 2016 Nutrition and Supplement Facts label rule. Because biological activity per IU varies by chemical form, IU alone could mislead consumers, so the FDA moved to an absolute mass unit (mcg or mg) tied directly to the Reference Daily Intake.
Why does vitamin A have four different conversion factors for one IU?
Vitamin A's IU was originally defined by retinol's biological activity. Beta-carotene and other carotenoids convert to retinol inside the body at different, form-dependent efficiencies, so the FDA assigns separate RAE factors to preformed vitamin A, supplemental beta-carotene, dietary beta-carotene, and other food provitamin A carotenoids.
Why do natural and synthetic vitamin E use different factors (0.67 vs 0.45)?
Natural RRR-alpha-tocopherol is a single stereoisomer that the body retains and uses more efficiently. The synthetic all-rac mixture contains multiple stereoisomers of lower biological activity, so the same IU amount converts to fewer mg.
Is the IU-to-mcg factor the same for vitamin D2 and D3?
Yes. FDA guidance sets both D2 and D3 at 1 mcg = 40 IU (factor 0.025), even though the two forms are metabolized somewhat differently in the body.
What RDI does the %DV on this page use?
The FDA's 2016 Reference Daily Intakes: 20 mcg for vitamin D, 900 mcg RAE for vitamin A, and 15 mg alpha-tocopherol for vitamin E — these replaced the older 1968-based values used before the 2016 label reform.
This tool applies only the FDA's official 2016 label conversion factors for vitamins D, A and E; it does not calculate absorption, bioavailability, drug interactions, or safe upper intake limits, and it is not medical advice. Confirm dosing with a product label or healthcare professional.
Sources: FDA — 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)