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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.