Mcg to IU Converter
Mcg to IU has no single answer, because the factor changes with the nutrient. For vitamin D, 1 mcg is 40 IU. Pick the nutrient and its form below, then type in either box.
1,000 IU × 0.025 = 25 mcg. Exact math, no rounding.
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Vitamin D: IU to mcg values
- 100 IU = 2.5 mcg
- 200 IU = 5 mcg
- 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
Label number conversions only (IU × 0.025), not amounts to take.
Why each nutrient has its own factor
IU stands for International Unit. An IU is not a weight. It is based on how active a nutrient is in tests. So one IU of vitamin D and one IU of vitamin E weigh very different amounts.
US labels moved from IU to weight units. The FDA now asks for vitamin D in mcg, vitamin A in mcg RAE and vitamin E in mg. In 2019 it published the factors that turn old IU values into the new units. The converter above uses those factors and nothing else.
The FDA factors
To go from IU to the label unit, multiply by the factor. To go back, divide.
| Nutrient and form | IU × factor | Gives |
|---|---|---|
| Vitamin D (D2 or D3) | 0.025 | mcg |
| Vitamin A, retinol | 0.3 | mcg RAE |
| Vitamin A, beta carotene in a supplement | 0.3 | mcg RAE |
| Vitamin A, beta carotene from food | 0.05 | mcg RAE |
| Vitamin A, other carotenoids from food | 0.025 | mcg RAE |
| Vitamin E, natural (RRR-alpha-tocopherol) | 0.67 | mg |
| Vitamin E, synthetic (all-rac-alpha-tocopherol) | 0.45 | mg |
Vitamin D, worked through
Vitamin D is the easy one. The FDA says 1 mcg is 40 IU, which is the same as IU × 0.025.
- IU to mcg: 1,000 IU × 0.025 = 25 mcg. This is the FDA's own example.
- mcg to IU: 50 mcg × 40 = 2,000 IU.
The FDA treats D2 and D3 the same for this math. On a US label, vitamin D must be shown in mcg. The IU number may follow in brackets, like "25 mcg (1,000 IU)". The supplement label guide walks through the rest of the panel.
Vitamin A and vitamin E
Vitamin A uses mcg RAE, which stands for retinol activity equivalents. The factor depends on the form. For retinol, 500 IU × 0.3 = 150 mcg RAE. If a product mixes forms, there is no one factor, the FDA says. You would need each form's amount.
Vitamin E uses mg of alpha-tocopherol. Natural and synthetic forms have different factors. For the natural form, 100 IU × 0.67 = 67 mg. For the synthetic form, 100 IU × 0.45 = 45 mg.
The converter rounds to 2 decimal places only when you go from mcg or mg back to IU, and it says so when it does.
What this page does not do
These factors are for label values of vitamins D, A and E only. Other things measured in IU use their own scales, and this site does not convert them. The numbers here are label math. They are not amounts to take.
For plain weight units, use the mg to mcg converter or the mcg to mg converter. To see what mcg means, read what is mcg.
Questions
How do you convert IU to mcg?
Multiply the IU by the factor for that nutrient. For vitamin D the factor is 0.025, so 400 IU × 0.025 = 10 mcg. Vitamin A and vitamin E use other factors.
What is 1,000 IU of vitamin D in mcg?
1,000 IU of vitamin D is 25 mcg, because 1,000 × 0.025 = 25.
What is 50 mcg of vitamin D in IU?
50 mcg of vitamin D is 2,000 IU, because 1 mcg of vitamin D is 40 IU.
How do you convert vitamin A IU to mcg?
Multiply by the factor for the form. For retinol or beta carotene in a supplement it is 0.3, so 1,000 IU is 300 mcg RAE. Beta carotene from food uses 0.05.
How do you convert vitamin E IU to mg?
Multiply by 0.67 for natural vitamin E or 0.45 for synthetic vitamin E. So 100 IU is 67 mg natural or 45 mg synthetic.
Is there one IU to mg factor for everything?
No. An IU is based on activity, not weight, so each nutrient has its own factor. This site only covers vitamins D, A and E.
What does IU stand for?
IU stands for International Unit. It measures activity, not weight, which is why it needs a factor to become mcg or mg.
Sources
- The International System of Units (SI), NIST Special Publication 330, 2019 edition, Table 7 SI prefixes and Table 8 non-SI units such as the liter, National Institute of Standards and Technology (NIST), doi:10.6028/NIST.SP.330-2019, read 2026-10-04
- Converting Units of Measure for Folate, Niacin, and Vitamins A, D, and E on the Nutrition and Supplement Facts Labels, US Food and Drug Administration, August 2019, read 2026-10-04
- 21 CFR 101.36, Nutrition labeling of dietary supplements, Electronic Code of Federal Regulations, read 2026-10-04