IU to mg conversion factors
An international unit measures biological activity, so each substance has its own IU. The FDA publishes the factors used to move Nutrition and Supplement Facts labels from IU to metric units.[1]
| Substance | Multiply IU by | Result in | FDA worked example |
|---|---|---|---|
| Vitamin D (D2 or D3) | 0.025 | mcg | 1,000 IU → 25 mcg |
| Vitamin E, natural (RRR-α-tocopherol) | 0.67 | mg | 3 IU → 2.01 mg |
| Vitamin E, synthetic (all-rac-α-tocopherol) | 0.45 | mg | 35 IU → 15.75 mg |
| Vitamin A, retinol or supplemental β-carotene | 0.3 | mcg RAE | 500 IU → 150 mcg RAE |
| Vitamin A, dietary β-carotene | 0.05 | mcg RAE | – |
For vitamin A, the FDA states there is no direct factor when a label's IU mixes retinol and carotenoids; the factors apply only to a known single form.[1]
Vitamin D: IU to mcg
1 mcg of vitamin D equals 40 IU, so multiply IU by 0.025.[1]
| IU | mcg | mg |
|---|---|---|
| 100 IU | 2.5 | 0.0025 |
| 400 IU | 10 | 0.01 |
| 600 IU | 15 | 0.015 |
| 800 IU | 20 | 0.02 |
| 1,000 IU | 25 | 0.025 |
| 2,000 IU | 50 | 0.05 |
| 4,000 IU | 100 | 0.1 |
| 5,000 IU | 125 | 0.125 |
Vitamin E: natural vs synthetic
Natural vitamin E is more active per mg than the synthetic form: 1 IU is 0.67 mg natural or 0.45 mg synthetic, and 1 mg of label vitamin E equals 2 mg of synthetic all-rac-α-tocopherol.[1] Check the form on the label before converting.
| IU | Natural (mg) | Synthetic (mg) |
|---|---|---|
| 10 IU | 6.7 | 4.5 |
| 15 IU | 10.05 | 6.75 |
| 30 IU | 20.1 | 13.5 |
| 100 IU | 67 | 45 |
| 200 IU | 134 | 90 |
| 400 IU | 268 | 180 |
IU to mL: insulin units
For insulin, units convert to volume, not to mg. U-100 insulin has 100 units in each mL, so units ÷ 100 = mL;[2] U-40 veterinary insulin has 40 units per mL, so units ÷ 40 = mL.[3] Charts for both are on units to mL. For a vitamin solution, IU to mL needs the IU per mL printed on that product.
Common IU mistakes
- Using one factor for everything. 1,000 IU is 25 mcg of vitamin D but 450 mg of synthetic vitamin E.
- Mixing natural and synthetic vitamin E. The factors differ by about half.
- Writing “IU”. ISMP lists “IU” as an error-prone abbreviation and advises spelling out “units”.[4]
- Confusing syringe units with IU. A syringe unit is a 0.01 mL line; see units to mg.
How we checked these numbers
Every result on this page comes from the same code as the calculator, and the known answers below are run as tests before each release.
| Input | Expected | Basis |
|---|---|---|
| 1,000 IU vitamin D | 25 mcg | FDA example 15 |
| 100 IU vitamin D | 2.5 mcg | FDA example 14 |
| 35 IU synthetic vitamin E | 15.75 mg | FDA example 19 |
| 3 IU natural vitamin E | 2.01 mg | FDA example 18 |
| 500 IU vitamin A (retinol) | 150 mcg RAE | FDA example 12 |
| 3,500 IU vitamin A (β-carotene in oil) | 1,050 mcg RAE | FDA example 13 |
Known limits.
- Covers only the substances listed. Other vitamins, hormones and biologics have their own IU definitions.
- Vitamin A mixtures cannot be converted with a single factor.
Change log.
- : Converter published with FDA 2019 factors; all six FDA worked examples reproduced.
Frequently asked questions
How many mg is 1,000 IU?
It depends on the substance: 0.025 mg (25 mcg) of vitamin D, 670 mg of natural vitamin E, or 450 mg of synthetic vitamin E.
How many mcg is 400 IU of vitamin D?
10 mcg. Multiply IU by 0.025.
How do you convert IU to mL?
For U-100 insulin, divide units by 100. For any other product you need its IU per mL from the label.
Are syringe units the same as IU?
No. Syringe units are 0.01 mL volume lines; they equal insulin units only for U-100 insulin in a U-100 syringe.