IU to mg Conversion Calculator
Choose a nutrient, enter a value, and convert instantly.
What does IU mean, and why convert to mg?
IU stands for International Unit. It measures biological activity, not pure weight. Milligrams (mg), on the other hand, measure mass. Because of that, IU and mg are only interchangeable when you know the exact substance and conversion factor.
This calculator helps you convert IU to mg (and mg to IU) for common vitamins where accepted factors are available.
Quick conversion factors used in this calculator
| Substance | 1 IU equals (mg) | 1 IU equals (mcg) |
|---|---|---|
| Vitamin A (retinol) | 0.0003 mg | 0.3 mcg |
| Vitamin D (D2/D3) | 0.000025 mg | 0.025 mcg |
| Vitamin E (natural d-alpha-tocopherol) | 0.67 mg | 670 mcg |
| Vitamin E (synthetic dl-alpha-tocopherol) | 0.45 mg | 450 mcg |
How to use the IU to mg calculator
- Select the nutrient form from the dropdown.
- Choose IU → mg or mg → IU.
- Enter your value.
- Click Calculate to see the converted result.
Example conversions
Vitamin D: 2,000 IU × 0.000025 mg/IU = 0.05 mg (50 mcg)
Vitamin A (retinol): 5,000 IU × 0.0003 mg/IU = 1.5 mg
Vitamin E natural: 400 IU × 0.67 mg/IU = 268 mg
Important notes about IU conversions
1) Nutrient form matters
For vitamin E, natural and synthetic forms have different conversion factors. The same IU value can mean different mg values depending on form.
2) Label units may vary
Modern supplement labels often use mcg or mg directly, especially for vitamin D and vitamin A. Older labels may still use IU. This tool helps bridge both systems.
3) Don’t generalize IU across all compounds
You cannot apply one IU-to-mg ratio to every vitamin, hormone, or medication. IU values are substance-specific.
Frequently asked questions
Is IU bigger than mg?
They measure different things. IU measures biological activity, while mg measures mass. One is not universally “bigger” than the other.
Can I convert IU to mg for any supplement?
Only if a validated conversion factor exists for that exact ingredient and form.
Why does vitamin E have multiple IU conversions?
The biological potency differs by chemical form, so natural and synthetic vitamin E use different IU-to-mg relationships.
Bottom line
The best way to convert IU to mg is to use the correct factor for the exact nutrient form. Use the calculator above for fast, clear conversions, and always confirm dosing with a qualified healthcare professional when health decisions are involved.