mcg to IU Converter
Convert micrograms (mcg) to International Units (IU) or back. Pick the nutrient — vitamin D, A (retinol), or E (natural or synthetic) — because each has its own USP conversion factor, then enter an amount to get the equivalent.
Example: with Amount 25 · Nutrient Vitamin D2/D3 · Direction mcg → IU → Result: 1,000 IU.
- In words25 mcg of Vitamin D2/D3 = 1,000 IU
- Conversion factorVitamin D2/D3: 1 IU = 0.025 mcg (40 IU/mcg)
Computed by the calculator below using its default values. Change any input to see your own numbers.
IU is a unit of biological activity, not mass, so the mcg factor depends entirely on the nutrient: vitamin D uses 40 IU per mcg (1 IU = 0.025 mcg).
Why mcg and IU need a nutrient
An International Unit measures biological activity, not weight, so there is no single mcg-to-IU number that works for everything. Each substance has a defined USP equivalence: 1 IU of vitamin D is 0.025 mcg of cholecalciferol or ergocalciferol, which is why 25 mcg equals 1,000 IU and 50 mcg equals 2,000 IU. Vitamin A and vitamin E use completely different factors, and vitamin E even splits by form — natural d-alpha-tocopherol is more active per milligram than the synthetic dl-alpha version.
That is the trap on supplement labels: a bottle listing 25 mcg of vitamin D and an older one listing 1,000 IU contain the same dose. Pick the nutrient first, then the direction, and the factor is applied for you.
How it’s calculated
Factors are USP/NIH standard: vitamin D, 1 IU = 0.025 mcg, so IU = mcg × 40. Vitamin A (retinol), 1 IU = 0.3 mcg, so IU = mcg ÷ 0.3 (× 3.333). Vitamin E natural (d-alpha-tocopherol), 1 IU = 0.667 mg, so IU = mg × 1.49. Vitamin E synthetic (dl-alpha-tocopherol), 1 IU = 1 mg, so IU = mg × 1.0. Reverse direction divides instead of multiplies.
IU reflects biological activity for a specific compound; these factors do not transfer between nutrients or to other IU-dosed substances (insulin, heparin, some antibiotics), which have their own definitions. Educational conversion, not dosing advice.
USP conversion factors
| Nutrient | 1 IU equals | To convert |
|---|---|---|
| Vitamin D2 / D3 | 0.025 mcg | mcg × 40 = IU |
| Vitamin A (retinol) | 0.3 mcg | mcg × 3.33 = IU |
| Vitamin A (beta-carotene) | 0.6 mcg | mcg × 1.67 = IU |
| Vitamin E (natural, d-alpha) | 0.667 mg | mg × 1.49 = IU |
| Vitamin E (synthetic, dl-alpha) | 1 mg | mg × 1.0 = IU |
USP International Unit definitions; NIH Office of Dietary Supplements vitamin fact sheets.
Common mistakes
- Using the vitamin D factor (40) for vitamin A or E — each nutrient has its own IU definition.
- Treating natural and synthetic vitamin E as identical; d-alpha-tocopherol is about 1.49 IU/mg versus 1.0 IU/mg for dl-alpha.
- Confusing mcg and mg — vitamin E factors are per milligram, so 400 IU natural E is about 268 mg, not mcg.
- Assuming a bottle in mcg is a lower dose than one in IU when they can be the same (25 mcg D = 1,000 IU).
Frequently asked questions
How do I convert mcg of vitamin D to IU?
Multiply micrograms by 40, because 1 IU of vitamin D is defined as 0.025 mcg. So 25 mcg is 1,000 IU and 50 mcg is 2,000 IU. Divide by 40 to go from IU back to mcg.
Why is there no single mcg-to-IU factor?
An IU measures biological activity for one specific substance, not mass. Vitamin D, A, and E each have a different amount of activity per microgram, so each gets its own conversion factor.
What is the difference between natural and synthetic vitamin E in IU?
Natural d-alpha-tocopherol is about 1.49 IU per mg, while synthetic dl-alpha-tocopherol is 1.0 IU per mg. The same milligram amount of natural E therefore carries more IU.
Can I use this to decide my supplement dose?
No. This only converts units. How much of any vitamin is right for you — and safe upper limits — is a medical decision, so confirm dosing with your doctor or pharmacist.