How it's calculated
IU = mcg × f
mcg = micrograms, IU = international units, f = conversion factor per nutrient (vitamin D 40, vitamin A retinol 3.333, vitamin E natural 0.001493). Reverse: mcg = IU ÷ f.
Common values
| Vitamin | 1 mcg equals |
|---|---|
| D | 40 IU |
| A (RAE) | 3.33 IU |
| E (natural) | 1.49 IU per mg |
Common questions
How many IU is 25 mcg of vitamin D?
25 mcg equals 1,000 IU, since each microgram is 40 IU. That's a common supplement dose.
Why does each vitamin have a different factor?
IU is defined by biological activity, which differs by compound. So vitamins A, D, and E each have their own mcg-to-IU conversion.