If you've ever been shopping down a long aisle full of vitamin supplements, you may have noticed that while some of them are labeled in the metric system (grams, milligrams or even micrograms), others are labeled in something called "IU." The IU stands for international units, an internationally accepted method of quantifying the biological effect you can expect from a dose of fat-soluble vitamins.
You can convert between mg or other metric units and IU, but each form of a given vitamin will have its own conversion factor.
According to Merriam-Webster, the precise international unit or IU definition is "a quantity of a biologically active substance (such as a vitamin) that produces a particular biological effect agreed upon as an international standard." Or, to put it another way, international units are a way of quantifying the biological effect you can expect from a vitamin dose.
Why Use the IU Measurement?
Each of these vitamin forms has a different level of biological activity or, to put it another way, you need different quantities of each form to achieve the same biological effect. Because of that, you can't standardize recommended doses by the weight (mcg, mg or g) of each vitamin, because the same quantity in one form would achieve a somewhat different biological effect than the same weight in another form.
So instead of standardizing weights that would achieve different effects, authorities decided to quantify the effect into something called an IU, or international unit. You can convert IU into weight measurements like mcg, mg or g, although the conversion factor is different for each form of the vitamin. Said conversion factors were laid out in the early 1900s publication International Conference for the Unification of the Formulae of Potent Medicaments.
Meanwhile, measuring the vitamins' effects in IU gives consumers and medical professionals a useful, internationally agreed-upon way of describing doses that will achieve the desired effect.
Recommended IU Vitamin Intake
Although you can convert IU measurements to units of weight, in most cases you don't need to do that conversion, which in turn would force you to be aware of which form of the vitamin you're consuming. Remember, the "IU" unit was created specifically to make standardized dosage effects easier.
So for fat-soluble vitamins, certain hormones and some enzymes, the recommended daily dosage and commercially available supplements are all usually labeled with IU. The fat-soluble vitamins you'll typically see labeled with IU, and their recommended daily intake for adults, is:
Other vitamins, such as vitamin C, are measured in milligrams, or mg, because they don't require the additional standardization imposed by the IU unit. Vitamins that are usually present or consumed in very small quantities, such as vitamin B12, are measured in micrograms.
Converting to/From International Units
As Dr. William C. Shiel Jr. explains on MedicineNet, "The definition of an international unit (IU) is generally arbitrary, technical, and eminently forgettable." And the specific definition varies between vitamins, hormones or whatever else is being measured.
But, every once in a while, you might find yourself in a position to convert between the IU measurement and milligrams, micrograms or grams. To make those calculations, you need to know which form of the vitamin you're dealing with and the appropriate conversion factor. If you're converting from IU to weight (mcg, mg or g), you'll divide by the conversion factor. If you're converting from weight to IU, you'll multiply by the conversion factor.
Converting Vitamin A From IU
For example, if you're dealing with vitamin A as retinol, the appropriate conversion factor is 3.33. If you have 8,000 IU of vitamin A and want to know what that is in weight, you'd use a table or an online search to look up the appropriate conversion factor (3.33 IU/mcg) and the usual unit of weight for measuring vitamin A (mcg). Then you'd divide the quantity of IU by the conversion factor:
8,000 IU ÷ 3.33 IU/mcg = 2,402 mcg
So, 8,000 IU of vitamin A as retinol is equivalent to 2,402 mcg of retinol.
But don't forget: Different forms of the same vitamin won't have identical conversion factors. If you're dealing with vitamin A as beta-carotene, the conversion factor is 1.66 IU/mcg. Once you do the math, this gives you:
8,000 IU ÷ 1.66 IU/mcg = 4,819 mcg
So, 8,000 IU of vitamin A as beta-carotene is equivalent to 4,819 mcg of beta-carotene.
Converting Vitamin D to/From IU
Vitamin D is another fat-soluble vitamin that is almost always expressed in IU, although some manufacturers list both IU and mcg on their supplement labels. If the vitamin D is present as cholecalciferol or ergocalciferol, the conversion rate is 40 IU/mcg.
So, if you have pills that give you 2,000 IU of vitamin D, divide by the conversion rate to find the equivalent in mcg:
2,000 IU ÷ 40 IU/mcg = 50 mcg
If you want to convert the other way, simply multiply by the conversion factor instead of dividing. For example, if you have 25 mcg of vitamin D as cholecalciferol and want to know how many IU that's equivalent to, you'd multiply:
25 mcg × 40 IU/mcg = 1,000 IU
So 25 mcg of vitamin D as cholecalciferol is equivalent to 1,000 IU.
Converting Vitamin E From IU
Like vitamin A, vitamin E may be sold in multiple forms, and the conversion factor depends on which form you've encountered. The conversion factor for vitamin E as dl-Alpha-tocopherol is 0.9 IU/mg, while the conversion factor for d-Alpha-tocopherol is 0.67 IU/mg.
So if you have 20 IU of vitamin E and want to know how many mg that is in each form, you'd divide by the appropriate conversion factor:
20 IU ÷ 0.9 IU/mg = 22.22 mg of dl-Alpha-tocopherol
20 IU ÷ 0.67 IU/mg = 29.85 mg d-Alpha-tocopherol
- U.S. Department of Health and Human Services: Dietary Guidelines for Americans, 2015-2020: Appendix 7
- Dietary Supplement Ingredient Database: MVM Calculator Frequently Asked Questions (FAQ)
- MedicineNet: Medical Definition of IU (International Unit)
- Merriam-Webster: International Unit
- Hathi Trust Digital Library: International Conference for the Unification of the Formulae of Potent Medicaments
- National Institutes of Health: Vitamin E