There is no constant conversion rate between IU, or International Units, and mg, or milligrams. International Units are summary units designed to express the total amount of a vitamin present in a supplement, even if that vitamin is present in various forms, with various concentrations.
The value of one IU for a particular substance is set by international agreement. For example, one IU of vitamin A is equal to either .0003 mg of retinol, or .0006 mg of beta-carotene, according to the NIH. A supplement containing .00015 mg of retinol and .0003 mg of beta-carotene, then, would contain one IU of vitamin A. The FAQ section of the National Institutes of Health's website contains a conversion table for some common types of vitamins.