Many consumers take vitamin and mineral supplements. The amount used for supplementation varies greatly depending on the particular vitamin or supplement. Common units of measure for vitamins and supplements are IU (international unit), milligrams (mg) and mcg (micrograms). IU defines the amount of a vitamin, mineral or drug that shows the same biological activity as a specified amount (weight) of the vitamin, supplement or drug. Milligrams and micrograms are units of weight or mass in the metric system.

Convert from milligrams to micrograms. By definition 1,000 mcg = 1 mg. Multiply the number of milligrams by 1,000 to find the micrograms. For example, convert 2.56 mg to micrograms: 2.56 X 1,000 = 2,560 mcg.

Divide the number of micrograms by 1,000 to find the number of milligrams. For example, convert 526 mcg to milligrams: 526 / 1,000 = 0.526 mg.

Determine the number of milligrams that a number of IUs represent. One IU represents an amount (milligrams) of the vitamin, supplement or drug that yields the same biological activity. This amount is different for every vitamin, supplement or drug, so there is no definite formula for this conversion, but the process is the same. For example, assume that 1 IU of vitamin E = 0.671 mg of vitamin E (d-alpha-tocopherol). Find the number of milligrams in 150 IUs of vitamin E. You can find the milligrams of vitamin E by multiplying the number of IUs by the equivalent amount of vitamin E in milligrams: 0.671 mg/IU X 150 IU = 100.65 mg of vitamin E.

Find the number of IUs that are equivalent to a defined amount (mg) of a vitamin, supplement or drug. Because you are dealing with IUs again, you require the same information as before to make any conversion. Assume you are working with vitamin E again but this time you need to find the number of IUs of vitamin E that equals 335.5 mg vitamin E. Substituting the known information into the following equation yields 335.5 mg / 0.671 mg/IU = 500 IU.