IU stands for international unit, which is a way of measuring the biological effects that a biologically active substance will have on the body. In the case of vitamin D, the recommended daily amount is listed in both micrograms (mcg) and international units (IU), and 1 mcg of vitamin D is equal to 40 IU. Therefore, to convert between mcg and IU, you can multiply the amount of vitamin D in mcg by 40 to get the amount in IU, or divide the amount in IU by 40 to get the amount in mcg. For example, 10 mcg of vitamin D is equal to 400 IU, and 800 IU of vitamin D is equal to 20 mcg. Its important to note that taking too much vitamin D can be harmful, and the recommended daily amount for adults up to age 70 is 600-800 IU.