IU to mg converter
There is no single IU-to-mg factor. An International Unit measures biological activity, so each substance has its own. For vitamin D, 1,000 IU = 0.025 mg (25 mcg). For natural vitamin E, 1,000 IU = 670 mg.
What one IU equals
These are the factors the converter uses. Each one is specific to its substance and comes from the source listed.
| Substance | 1 IU = | 1,000 IU = | Source |
|---|---|---|---|
| Vitamin D (D3 or D2) | 0.025 mcg | 25 mcg | NIH ODS |
| Vitamin E, natural (d-alpha) | 0.67 mg | 670 mg | NIH ODS |
| Vitamin E, synthetic (dl-alpha) | 0.45 mg | 450 mg | NIH ODS |
| Vitamin A as retinol | 0.3 mcg RAE | 300 mcg RAE | NIH ODS |
| Beta-carotene, supplements | 0.3 mcg RAE | 300 mcg RAE | NIH ODS |
| Beta-carotene, food | 0.05 mcg RAE | 50 mcg RAE | NIH ODS |
| Insulin, human | 0.0347 mg | 34.7 mg | WHO standard |
| Heparin | no fixed factor | — | USP |
How to convert IU to mg
amount = IU × (what 1 IU equals for that substance) vitamin D: 5,000 IU × 0.025 mcg = 125 mcg = 0.125 mg vitamin E natural: 400 IU × 0.67 mg = 268 mg
To go from mg (or mcg) back to IU, divide by the same factor. NIH gives the reverse factors for vitamin E directly: 1 mg is 1.49 IU of the natural form or 2.22 IU of the synthetic form.
Common IU conversions
- 400 IU to mg: vitamin D, 0.01 mg (10 mcg)
- 1,000 IU to mg: vitamin D, 0.025 mg (25 mcg)
- 2,000 IU to mg: vitamin D, 0.05 mg (50 mcg)
- 5,000 IU to mg: vitamin D, 0.125 mg (125 mcg)
- 50,000 IU to mg: vitamin D, 1.25 mg
- 400 IU vitamin E to mg: 268 mg natural or 180 mg synthetic
Why labels are moving away from IU
Since the FDA's updated Nutrition and Supplement Facts rules, US labels give vitamins D, E and A by weight (mcg or mg), and IU only in brackets if at all. Vitamin A moved to mcg RAE (retinol activity equivalents), which accounts for how much of each form the body turns into vitamin A. Older bottles and many prescriptions still use IU, which is why the conversion keeps coming up.
Questions
How many mg is 1,000 IU?
For vitamin D, 0.025 mg (25 mcg). For natural vitamin E, 670 mg; for synthetic vitamin E, 450 mg. For vitamin A as retinol, 0.3 mg (300 mcg RAE). The substance changes the answer completely.
How do I convert mg to IU?
Divide the amount by what one IU equals for that substance. For vitamin D, divide mcg by 0.025 (or multiply by 40). For natural vitamin E, divide mg by 0.67.
Why can't I convert heparin IU to mg?
Heparin is extracted from animal tissue and its activity per mg varies by batch, so it is dosed and labelled in units only. Use the units on the label.
Is IU the same as the units on an insulin syringe?
Not quite. For U-100 insulin, 1 IU of insulin happens to fill one syringe unit (0.01 mL). For any other liquid, syringe units measure volume only. See the mg to units calculator.
Is 1,000 IU of vitamin D a lot?
It's a common supplement strength. NIH's adult upper limit is 4,000 IU (100 mcg) a day. Ask your clinician what amount is right for you.