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The idea of using IUs to standardize the reporting of vitamin D potency was first established by the World Health Organization in 1931 using vitamin D2. Once vitamin D3 could be made by scientists, the IU recommendation was changed to be based off vitamin D3 in 1949. Today, many countries still use the IU to measure vitamin D; 1 IU of vitamin D is equivalent to 0.025 micrograms (abbreviated as either mcg or μg) of cholecalciferol or ergocalciferol.1 Conversely, 1 microgram of vitamin D equals 40 IU of vitamin D.

This calculator calculates the iu (international unit) using mcg (microgram (mcg)) values.