There is no single IU-to-mcg factor — it depends on the vitamin
An IU (International Unit) measures biological activity, not weight, so the same number of IU means a different mass for each vitamin. You cannot convert until you know which one you have. Pick the substance, then multiply.
The per-vitamin factors
| Vitamin / form | 1 IU = | To get IU |
|---|---|---|
| Vitamin D (D2 / D3) | 0.025 mcg | mcg × 40 |
| Vitamin A — retinol | 0.3 mcg RAE | mcg × 3.33 |
| Vitamin A — beta-carotene (supplement) | 0.6 mcg | mcg × 1.67 |
| Vitamin A — beta-carotene (food) | 1.2 mcg | mcg × 0.83 |
| Vitamin E — natural (d-) | 0.67 mg | mg × 1.49 |
| Vitamin E — synthetic (dl-) | 0.45 mg | mg × 2.22 |
Vitamin C never uses IU — it is stated directly in mg. Vitamin A uses mcg RAE (Retinol Activity Equivalents), which is why its forms each carry a different factor.
Vitamin D at common doses
Vitamin D is by far the most searched of these, and the easy rule is IU ÷ 40 = mcg.
| IU | mcg | Note |
|---|---|---|
| 400 IU | 10 mcg | infant daily amount |
| 600 IU | 15 mcg | adult RDA (ages 1–70) |
| 800 IU | 20 mcg | adult RDA (71+) |
| 2,000 IU | 50 mcg | common supplement |
| 4,000 IU | 100 mcg | tolerable upper limit |
| 5,000 IU | 125 mcg | OTC dose — above the upper limit |
The 4,000 IU (100 mcg) daily upper limit is the National Academies figure for adults; several common OTC pills sit above it.
The two traps to watch for
- One vitamin's factor on another. Using the vitamin D factor for vitamin A, or vice versa, is off by more than 10×. Always confirm the substance first.
- d- versus dl- on vitamin E. One letter changes the potency by roughly 50%. The natural "d-" form is stronger per IU than the synthetic "dl-" form, so 400 IU is 268 mg natural but only 180 mg synthetic.
Common questions
How many mcg is 1000 IU of vitamin D?
1000 IU of vitamin D is 25 mcg. The factor is 40 IU per 1 mcg, so divide IU by 40, or multiply mcg by 40 to go back.
Is there one IU-to-mcg conversion for all vitamins?
No, and this is the mistake to avoid. Each vitamin has its own factor. 1000 IU is 25 mcg for vitamin D but 300 mcg RAE for vitamin A retinol, because an IU measures biological activity, not mass.
Why does vitamin E convert to mg instead of mcg?
Vitamin E is dosed in milligrams. Natural vitamin E (d-alpha-tocopherol) is 0.67 mg per IU and the synthetic form (dl-alpha-tocopherol) is 0.45 mg per IU, so the same IU number can differ by about half depending on the form.
