What does “IU” mean on a vitamin D label?
When you look at the vitamin D label, you might notice that the amount of vitamin D is given as “IU” and not mg or mcg like many of the other vitamins. What does “IU” stand for and why is it used?
“IU” stands for International Unit. It is a unit of measure, but one that is very different than what we are accustomed to seeing on labels, such as the milligram (mg) or microgram (mcg). The mg and mcg units depict an amount based on mass or volume, something that we can literally see or feel. However, the IU measurement describes something that we cannot see; the potency, or biological activity of a product. This is particularly helpful to pharmacologists when products have more than one form, such is the case with vitamin D. Vitamin D has two different forms that are found in supplements: vitamin D2, also known as ergocalciferol, and vitamin D3, cholecalciferol. Each of these has a different biological activity or potency, so scientists need a reliable way to compare the potency of these two vitamins. Measuring vitamin D in IUs gives scientist a way to compare apples to apples.
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. Conversely, 1 microgram of vitamin D equals 40 IU of vitamin D.
You can use the following to convert vitamin D:
From IU to mcg: IU/40 = mcg
For example: 400 IU/40 = 10 mcg
From mcg to IU: mcg * 40 = IU
A simpler approach is to use the following vitamin D conversion table:
If you would like to understand vitamin D labels in the UK, read our post "What does MCG (µg) mean on a vitamin D label?"
Originally published on February 28, 2016
 The World Health Organization’s Expert Committee on Biological Standardization , Report of the Subcommittee on the Fat Soluble Vitamins, Lonon, 26-29 April 1949. http://apps.who.int/iris/bitstream/10665/38884/1/WHO_TRS_3.pdf