Last updated October 4, 2026 · Report an error
Converts between micrograms (mcg) and International Units (IU) for specific vitamins, using standard published conversion factors from the NIH Office of Dietary Supplements fact sheets. These factors are substance-specific and are not interchangeable between vitamins: Vitamin D is 1 mcg = 40 IU; Vitamin A (retinol) is 1 mcg RAE = 3.33 IU; Vitamin E (natural) is 1 mg = 1.49 IU; Vitamin E (synthetic) is 1 mg = 2.22 IU. This is a unit-conversion arithmetic tool, not a dosing recommendation. Consult a doctor or pharmacist for actual supplement dosing guidance.
How it works
Select the substance (Vitamin D, Vitamin A retinol, or Vitamin E natural/synthetic), the conversion direction, and enter your value. The tool applies the substance-specific NIH conversion factor.
- Enter substance.
- Enter conversion direction.
- Enter value to convert.
- Click Calculate to see your results.
Examples
25 mcg Vitamin D
25 x 40 = 1,000 IU.
15 mg Vitamin E, natural form
15 x 1.49 = 22.35 IU.
Who should use it
- Comparing supplement labels that use different units.
- General nutrition education.
Industry applications
- Nutrition and supplement education
- General wellness reference
Advantages
- Covers 3 commonly searched, well-established substances with clearly cited NIH factors.
- Works in both directions (mcg to IU and IU to mcg).
Limitations
- Does not cover every possible substance/form (e.g. beta-carotene) — scoped to the most standardized, commonly cited conversions.
Common mistakes to avoid
- Applying one vitamin's conversion factor to a different vitamin.
- Not checking whether a Vitamin E product is natural or synthetic form before converting.
Best practices
- Always confirm the exact substance and form (e.g. natural vs. synthetic Vitamin E) from the supplement label before converting.
Tips
- Supplement labels increasingly use mcg for Vitamin D per updated FDA labeling rules, while older products or other countries may still use IU — this tool helps compare the two.