What this calculator does
International Units measure biological activity rather than mass, so the conversion to micrograms differs for every substance. Vitamin D converts at 40 IU per microgram; vitamin A as retinol at 3.33.
Supplement labels use both conventions, often inconsistently. A 25 mcg vitamin D capsule and a 1,000 IU one contain exactly the same amount, which is not obvious from the packaging.
The formula
The selected vitamin sets the conversion factor, which is then applied in whichever direction is chosen. The factors are specific to each vitamin and are not interchangeable.
| Term | Meaning |
|---|---|
| International Unit | A measure of biological activity, with a different mass equivalent for each substance. |
| Vitamin D factor | 1 microgram equals 40 IU. |
| Vitamin A factor | 1 microgram of retinol equals 3.33 IU. |
The inputs explained
| Field | What to enter |
|---|---|
| Vitamin | Which vitamin, since each has its own factor. |
| Direction | Which direction to convert in. |
| Amount | The amount to convert. |
When to use it
Comparing two supplements
One label in IU and one in micrograms cannot be compared without converting.
Checking a dose against guidance
Recommendations and labels often use different units.
Reading older packaging
IU was the older convention and remains common on supplements.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
What do common vitamin D doses convert to?
Three micrograms amounts converted to IU.
| Amount in mcg | Converted amount | IU amount |
|---|---|---|
| 10 mcg | 400.00 IU | 400.00 IU |
| 25 mcg | 1,000.00 IU | 1,000.00 IU |
| 50 mcg | 2,000.00 IU | 2,000.00 IU |
Questions
Why do International Units exist at all?
Because they were introduced before many vitamins could be isolated and weighed accurately. Defining a unit by biological effect allowed doses to be standardised when the chemistry was not yet settled.
Why does each vitamin have a different factor?
Because the factor reflects biological potency rather than mass. A microgram of vitamin D and a microgram of vitamin A produce quite different amounts of biological activity, so their unit equivalences differ.
Is vitamin A always 3.33 IU per microgram?
Only for retinol. Beta-carotene and other provitamin A carotenoids convert far less efficiently, which is why modern guidance uses retinol activity equivalents rather than IU for vitamin A.
Which unit should I use?
Micrograms, where possible. Most modern guidance and labelling regulations have moved to mass units precisely because IU is a source of confusion. IU persists mainly on older products.
For cholesterol unit conversion, see the cholesterol units calculator. For blood glucose units, see the blood glucose converter.