IU stands for International Units, an internationally accepted method of quantifying the biological effect you can expect from a dose of fat-soluble vitamins. You can convert between mg or other metric units and IU, but each form of a given vitamin will have its own conversion factor.For Vitamin D3, the conversion rate is 40 IU/mcg. For example if you have 5000 IU of Vitamin D3, divide by the conversion rate to find the equivalent in mcg: 5000 IU ÷ 40 IU/mcg = 125 mcg