Interactive mg ↔ IU Converter
Use this calculator for common vitamin conversions. Because IU measures biological activity, your conversion depends on the exact nutrient form.
Educational use only. Always verify supplement dosing with a healthcare professional.
What this mg to IU calculator does
This mg to IU calculator helps you convert supplement amounts between milligrams (mg) and International Units (IU) for selected vitamins. It is especially useful when one label shows mg and another shows IU, and you want to compare doses accurately.
The most important rule is simple: there is no single universal mg-to-IU conversion. Every nutrient has its own conversion factor, and in some cases, different chemical forms of the same nutrient use different factors.
mg vs IU: Why conversions can be confusing
Milligram (mg)
Milligram is a unit of mass. It tells you how much material is present by weight. For example, 1 mg means one-thousandth of a gram.
International Unit (IU)
IU is a standardized measure of biological effect or potency, not just weight. That means 1 IU of one vitamin is not equivalent to 1 IU of another vitamin in terms of mass.
Because IU is activity-based, conversion from IU to mg or mg to IU requires the exact nutrient and form.
Common conversion factors used in this calculator
| Nutrient form | IU per mg | Helpful relationship |
|---|---|---|
| Vitamin A (Retinol) | 3,333.33 IU/mg | 1 IU = 0.3 mcg retinol |
| Vitamin A (Beta-carotene, supplements) | 1,666.67 IU/mg | 1 IU = 0.6 mcg beta-carotene |
| Vitamin A (Beta-carotene, food) | 833.33 IU/mg | 1 IU = 1.2 mcg beta-carotene |
| Vitamin D2/D3 | 40,000 IU/mg | 1 IU = 0.025 mcg vitamin D |
| Vitamin E (Natural d-alpha-tocopherol) | 1.4925 IU/mg | 1 IU = 0.67 mg natural form |
| Vitamin E (Synthetic dl-alpha-tocopherol) | 2.2222 IU/mg | 1 IU = 0.45 mg synthetic form |
How to use the calculator
- Select whether you want mg to IU or IU to mg.
- Choose the exact vitamin form from the dropdown.
- Enter your amount (for example, 0.025 mg or 2000 IU).
- Click Calculate to see the converted value and formula used.
This is useful when checking a vitamin D dosage, comparing vitamin E softgels, or translating old IU-based labels into mg values.
Worked examples
Example 1: Vitamin D3, 0.05 mg to IU
Vitamin D uses 40,000 IU per mg. So:
IU = 0.05 × 40,000 = 2,000 IU
Example 2: Vitamin A retinol, 5,000 IU to mg
Retinol uses 3,333.33 IU per mg. So:
mg = 5,000 ÷ 3,333.33 ≈ 1.5 mg
Example 3: Natural vitamin E, 30 IU to mg
Natural vitamin E uses 1.4925 IU per mg. So:
mg = 30 ÷ 1.4925 ≈ 20.1 mg
Important label-reading tips
- Always match the form (for example, natural vs synthetic vitamin E).
- Check whether the label uses mcg, mg, or IU.
- Some modern labels use mcg RAE (vitamin A) instead of IU.
- Do not assume a conversion for nutrients not standardized in IU.
- For prescription products, follow clinician instructions over online calculators.
Frequently asked questions
Can I convert any supplement from mg to IU?
No. IU is only defined for specific compounds where biological activity standards exist. Many nutrients and medications do not use IU at all.
Why are there different vitamin A values?
Because vitamin A activity depends on chemical form and source. Retinol and beta-carotene do not have the same potency per unit mass.
Is vitamin D conversion the same for D2 and D3?
For IU labeling, both are typically converted using 1 IU = 0.025 mcg (40 IU per mcg, or 40,000 IU per mg).
Is this a medical dosing tool?
No. This calculator is for educational and label-comparison use. For deficiency treatment, pregnancy, pediatric dosing, or chronic illness, consult a qualified healthcare professional.
Bottom line
A good mg to IU calculator saves time and reduces label confusion, but only when paired with the correct nutrient form. Use the tool above to convert quickly, then double-check your supplement facts panel and dosing instructions before making changes.