Important: IU is not a universal mass unit. The correct conversion depends on the exact substance form.
What this IU to milligrams calculator does
This calculator converts International Units (IU) into milligrams (mg) for common vitamins and supplement forms. IU measures biological activity, while mg measures physical mass. Because of that, there is no single conversion that works for every nutrient.
In practice, you must pick the specific vitamin form first (for example, vitamin D3 vs vitamin E natural), then apply the right factor. The tool above handles this automatically and also supports a custom factor when needed.
IU vs mg: why the conversion changes
International Units are effect-based
IU was designed to represent biological potency. Different substances can produce similar biological effects at different masses, so the IU value does not map to one fixed mg number across all vitamins.
Milligrams are mass-based
Milligrams are straightforward weight units: 1 mg is always 1 mg. But the number of IU in 1 mg depends on the chemistry and potency of the specific compound.
milligrams (mg) = IU × (mg per IU)
Common IU to mg factors used in this calculator
- Vitamin D3: 1 IU = 0.000025 mg (0.025 mcg)
- Vitamin D2: 1 IU = 0.000025 mg (0.025 mcg)
- Vitamin A (retinol): 1 IU = 0.0003 mg (0.3 mcg)
- Vitamin A (beta-carotene supplement): 1 IU = 0.0006 mg (0.6 mcg)
- Vitamin E (natural d-alpha-tocopherol): 1 IU = 0.67 mg
- Vitamin E (synthetic dl-alpha-tocopherol): 1 IU = 0.45 mg
Quick examples
Example 1: 2,000 IU vitamin D3 to mg
2,000 × 0.000025 mg = 0.05 mg (which is 50 mcg).
Example 2: 5,000 IU vitamin A (retinol) to mg
5,000 × 0.0003 mg = 1.5 mg.
Example 3: 400 IU natural vitamin E to mg
400 × 0.67 mg = 268 mg.
How to use this calculator accurately
- Read your supplement label carefully and identify the exact nutrient form.
- Enter the IU value exactly as shown on the label.
- Select the matching option in the dropdown.
- Use custom factor only when you have a verified conversion source.
- Double-check units: mcg, mg, and IU are easy to mix up.
Frequently asked questions
Can I use one universal IU to mg number?
No. IU-to-mg conversion is nutrient-specific and often form-specific.
Why does vitamin E look so different from vitamin D?
Different compounds have different biological potencies relative to mass. That is exactly why IU exists in the first place.
Is this calculator medical advice?
No. This is an educational conversion tool. For dosing decisions, especially at high doses or with medical conditions, consult a licensed healthcare professional.
Bottom line
Converting IU to milligrams is simple once you have the correct factor for the exact substance. Use the calculator above to avoid manual mistakes and get quick, consistent conversions for common vitamins.