IU stands for International Unit, a standardised measure of biological activity used to compare potency across vitamins and medications. It's especially common in fat-soluble vitamins like Vitamin D, Vitamin A, and Vitamin E.

• IU reflects the effect of a substance in the body, not just its weight.
• mg (milligrams) and µg (micrograms) measure physical weight, regardless of potency.
  

For example, 1,000 IU of Vitamin D equals 25 micrograms. But for Vitamin E, 1,000 IU is 670 milligrams. The difference depends on the substance.

Most supplement packaging and health guidelines now use mg or µg instead of IU - especially after changes in EU and FDA labeling laws. This conversion tool helps you make sense of older IU values and compare products more easily.