MCG ⇄ IU Converter
Use this calculator to convert between micrograms (mcg) and International Units (IU) for common vitamins. Choose the nutrient first, because conversion factors are different.
Tip: IU is not used for every nutrient. Check your supplement label for the exact vitamin form before converting.
How to convert mcg to IU
Converting mcg to IU is simple once you know the nutrient-specific factor. International Units measure biological activity, while micrograms measure mass. Because different vitamins have different biological effects per microgram, there is no single universal conversion rate.
This is why a “mcg to IU” conversion for Vitamin D is not the same as Vitamin A or Vitamin E.
Common conversion factors
| Nutrient | mcg to IU Formula | IU to mcg Formula |
|---|---|---|
| Vitamin D (D2/D3) | IU = mcg × 40 | mcg = IU ÷ 40 |
| Vitamin A (retinol) | IU = mcg × 3.3333 | mcg = IU ÷ 3.3333 |
| Vitamin A (beta-carotene) | IU = mcg × 1.6667 | mcg = IU ÷ 1.6667 |
| Vitamin E (natural d-alpha) | IU = mcg × 0.0014925 | mcg = IU ÷ 0.0014925 |
| Vitamin E (synthetic dl-alpha) | IU = mcg × 0.0022222 | mcg = IU ÷ 0.0022222 |
Worked examples
Example 1: Vitamin D
If you have 25 mcg of Vitamin D:
- IU = 25 × 40
- IU = 1000 IU
Example 2: Vitamin A (retinol)
If a label lists 900 mcg Vitamin A (retinol):
- IU = 900 × 3.3333
- IU ≈ 3000 IU
Example 3: Reverse conversion
If a supplement has 2000 IU Vitamin D, convert to mcg:
- mcg = 2000 ÷ 40
- mcg = 50 mcg
Why your conversion might not match your label
Two major reasons explain most discrepancies:
- Different vitamin forms: For example, natural and synthetic Vitamin E have different IU relationships.
- Label standards changed: Many newer labels now favor mcg or mg instead of IU, especially outside older formulations.
FAQ
Is 1 mcg always equal to the same IU?
No. IU conversion is nutrient-specific and sometimes form-specific.
Can I use this for all vitamins and hormones?
No. Only nutrients with established IU equivalencies can be converted this way. Many nutrients should be tracked directly in mcg or mg.
What’s the fastest way to avoid mistakes?
Match three things before converting: nutrient name, chemical form, and direction (mcg to IU or IU to mcg).
Bottom line
A reliable mcg-to-IU calculation always starts with the right conversion factor. Use the calculator above, confirm the exact nutrient form on your label, and double-check values if you are managing a medical condition or therapeutic dose.