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IU to mcg Converter: Vitamin D, A and E
There is no single IU to mcg conversion, because an international unit (IU) measures biological activity and every substance has its own factor. For vitamin D, 1 mcg equals 40 IU, so 1,000 IU is 25 mcg. For vitamin E, 1 IU is 0.67 mg of natural or 0.45 mg of synthetic alpha-tocopherol. For vitamin A, 1 IU of retinol is 0.3 mcg RAE.
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IU factors differ for every substance. Only the ones NIH publishes are offered.
1000 IU equals
25mcg
That is also 0.025 mg.
Unit arithmetic from NIH Office of Dietary Supplements factors. Not a dose recommendation; check the product label.
Vitamin D: IU to mcg table
400 IU
10 mcg
600 IU
15 mcg
800 IU
20 mcg
1,000 IU
25 mcg
2,000 IU
50 mcg
4,000 IU
100 mcg
5,000 IU
125 mcg
10,000 IU
250 mcg
Converting blood-test results instead (ng/mL to nmol/L)? Use the lab unit converter.
Vitamin D IU to mcg: 1 mcg = 40 IU
The NIH Office of Dietary Supplements lists vitamin D amounts in both units and states that 1 mcg of vitamin D is equal to 40 IU. The formulas are:
- IU to mcg: mcg = IU ÷ 40
- mcg to IU: IU = mcg × 40
| IU | Math | mcg |
|---|---|---|
| 400 IU | 400 ÷ 40 | 10 mcg |
| 800 IU | 800 ÷ 40 | 20 mcg |
| 1,000 IU | 1,000 ÷ 40 | 25 mcg |
| 2,000 IU | 2,000 ÷ 40 | 50 mcg |
NIH also notes that the FDA Daily Value for vitamin D is 20 mcg (800 IU) for adults and children age 4 and older, and that labels list mcg with IU optional in parentheses.
IU to mg: vitamin E, A and why the answer changes
Asking for “IU to mg” has no single answer. These are the factors NIH publishes for the vitamins that use IU on labels.
| Substance | 1 IU equals |
|---|---|
| Vitamin D (D2 or D3) | 0.025 mcg |
| Vitamin E, natural (d-alpha-tocopherol) | 0.67 mg alpha-tocopherol |
| Vitamin E, synthetic (dl-alpha-tocopherol) | 0.45 mg alpha-tocopherol |
| Vitamin A, retinol or supplemental beta-carotene | 0.3 mcg RAE |
| Vitamin A, beta-carotene from food | 0.05 mcg RAE |
| Vitamin A, alpha-carotene or beta-cryptoxanthin from food | 0.025 mcg RAE |
Worked example, vitamin E:400 IU of natural vitamin E is 400 × 0.67 = 268 mg of alpha-tocopherol. 400 IU of the synthetic form is 400 × 0.45 = 180 mg. NIH's own example runs the other way: 15 mg of natural alpha-tocopherol is 15 × 1.49 = 22.4 IU.
Worked example, vitamin A: 5,000 IU of retinol is 5,000 × 0.3 = 1,500 mcg RAE, or 1.5 mg. NIH notes that RAE converts back to IU only when the source is known, for example 900 mcg RAE is 3,000 IU of retinol but 18,000 IU of beta-carotene from food.
What cannot be converted: insulin, hCG and other IUs
The converter refuses substances with no published factor rather than guessing. Choose “Something else” in the picker to see why. Insulin is the clearest case: conventional units are based on bioefficacy and SI units on mass, and a 2019 commentary in the Journal of Diabetes Science and Technology describes how two different conversion factors are in use, a gap of roughly 15%.
Units of botulinum toxin products are specific to each brand and are not interchangeable, so this site does not convert them. For reconstitution math from the label, see the Botox reconstitution calculator.
Related converters
Once you have mcg, the mg to mcg converter moves it to mg or g. For blood-test units such as 25-hydroxy vitamin D in ng/mL and nmol/L, use the lab unit converter.
Frequently asked questions
How do you convert vitamin D IU to mcg?
Divide the IU by 40. NIH states that 1 mcg of vitamin D equals 40 IU, so 1,000 IU ÷ 40 = 25 mcg and 800 IU ÷ 40 = 20 mcg. To go from mcg to IU, multiply by 40.
How do you convert IU to mg?
It depends on the substance. For vitamin E, multiply IU by 0.67 for the natural form or 0.45 for the synthetic form to get mg of alpha-tocopherol. For vitamin D and vitamin A the published factors are in mcg, so divide mcg by 1,000 to get mg. There is no universal IU to mg factor.
Why can't I convert any IU to mcg with one number?
An IU is defined by the biological effect of a specific substance, not by weight. 1 IU of vitamin D weighs 0.025 mcg, while 1 IU of vitamin E weighs more than 20,000 times as much. Each substance, and sometimes each form of it, needs its own factor.
How many mcg RAE is 1 IU of vitamin A?
It depends on the source. NIH lists 1 IU of retinol or of supplemental beta-carotene as 0.3 mcg RAE, 1 IU of dietary beta-carotene as 0.05 mcg RAE, and 1 IU of dietary alpha-carotene or beta-cryptoxanthin as 0.025 mcg RAE. Pick the matching source in the converter.
What is the difference between natural and synthetic vitamin E in IU?
Natural vitamin E (d-alpha-tocopherol) is 1 IU = 0.67 mg alpha-tocopherol, and synthetic (dl-alpha-tocopherol) is 1 IU = 0.45 mg. Going the other way, 1 mg of alpha-tocopherol is 1.49 IU of the natural form or 2.22 IU of the synthetic form. The label usually says which form the product uses.
Can I convert insulin or hCG units to mcg?
No, not with a generic factor. Insulin units are based on biological activity while SI units are based on mass, and published commentary notes that two different conversion factors circulate. For hCG, heparin and other biologics, the product label gives the activity per mL or per vial.
Does this tell me how much of a vitamin to take or give?
No. It only converts a number between units. Amounts for patients are a clinical decision for the treating provider, and the product label states what the product contains.
Sources and scope
- NIH Office of Dietary Supplements: Vitamin D Fact Sheet for Health Professionals
States 1 mcg vitamin D = 40 IU, and the Daily Value of 20 mcg (800 IU).
- NIH Office of Dietary Supplements: Vitamin A Fact Sheet for Health Professionals
IU to mcg RAE factors for retinol, supplemental and dietary beta-carotene, and alpha-carotene or beta-cryptoxanthin.
- NIH Office of Dietary Supplements: Vitamin E Fact Sheet for Health Professionals
IU to mg factors for natural (0.67) and synthetic (0.45) alpha-tocopherol, and the reverse factors.
- Knopp JL et al. Insulin Units and Conversion Factors: A Story of Truth, Boots, and Faster Half-Truths. J Diabetes Sci Technol, 2019 (PMC6501531)
Peer-reviewed commentary on why insulin IU and pmol/L conversion is not straightforward.
Unit arithmetic from published NIH factors, not medical advice. It offers only substances with a published factor, never converts between botulinum toxin products, and does not recommend an amount. Check the product label.