Micrograms (mcg) to IU Converter
Use this calculator to convert micrograms to International Units (IU) for common vitamins.
Why convert micrograms to IU?
Supplement labels can be confusing. Some products list vitamins in micrograms (mcg), while others use International Units (IU). This tool helps you quickly convert between those units for vitamins where IU is still common.
The important thing to know is this: IU is a measure of biological activity, not just mass. That means the conversion factor changes depending on the vitamin and even the chemical form of that vitamin.
Common micrograms to IU conversion factors
| Vitamin/Form | Conversion | IU per 1 mcg |
|---|---|---|
| Vitamin D (D2 or D3) | 1 mcg = 40 IU | 40 |
| Vitamin A (retinol) | 1 IU = 0.3 mcg retinol | 3.3333 |
| Vitamin A (beta-carotene) | 1 IU = 0.6 mcg beta-carotene | 1.6667 |
| Vitamin E (natural d-alpha-tocopherol) | 1 mg = 1.49 IU | 0.00149 |
| Vitamin E (synthetic dl-alpha-tocopherol) | 1 mg = 1.11 IU | 0.00111 |
How to use this calculator
- Enter your amount in micrograms.
- Choose the correct vitamin and form.
- Click Convert to IU to see your result instantly.
Example conversions
Example 1: Vitamin D
If you have 25 mcg of Vitamin D:
IU = 25 × 40 = 1000 IU.
Example 2: Vitamin A (retinol)
If you have 900 mcg of retinol:
IU = 900 × 3.3333 ≈ 3000 IU.
Example 3: Vitamin E (natural)
If you have 15000 mcg (15 mg) natural Vitamin E:
IU = 15000 × 0.00149 = 22.35 IU.
Important notes about accuracy
For Vitamin A, modern nutrition labeling often prefers mcg RAE (Retinol Activity Equivalents) rather than IU. If your healthcare provider gave guidance in RAE, use that standard first and only convert when needed for older labels.
For Vitamin E, labels may show mg alpha-tocopherol instead of IU. This is generally clearer and is increasingly preferred.
FAQ
Is IU the same as mcg?
No. IU measures biological effect, while mcg measures mass. They only convert with a vitamin-specific factor.
Can I use one conversion factor for all vitamins?
No. Each vitamin has its own conversion rules, and some have multiple factors depending on chemical form.
Should I supplement based on this calculator alone?
Use this calculator for unit conversion only. Dosing decisions should be made with a clinician, especially for fat-soluble vitamins (A, D, and E), where too much can be harmful.
Bottom line
This micrograms to IU calculator gives quick, practical conversions for common vitamin forms. It is ideal for comparing labels, checking old supplement recommendations, and understanding unit differences with confidence.