Mcg to IU Converter
Convert between micrograms (mcg) and international units (IU) for vitamins A, D, E, and more. Free online mcg to IU converter with substance-specific conversion factors and comparison chart.
About This Calculator
The Mcg to IU Converter helps you convert between micrograms (mcg) and international units (IU) for a wide range of vitamins and substances. Whether you are reading a supplement label from a different country or trying to match a recommended daily allowance, this calculator handles substance-specific conversion factors automatically. It supports vitamin D (D2 and D3), multiple forms of vitamin A (retinol, beta-carotene, retinyl palmitate, retinyl propionate, mixed provitamin carotenoids), natural and synthetic vitamin E, vitamin C, and insulin.
International units (IU) measure the biological activity of a substance rather than its mass. This means each substance has a unique IU-to-microgram ratio. For example, 1 mcg of vitamin D equals 40 IU, while 1 mcg of vitamin A (retinol) equals only 3.33 IU. Our calculator uses the internationally established conversion factors from the WHO International Biological Reference Preparations and the NIH Dietary Supplement Ingredient Database to ensure accuracy.
How to use it: Select your substance from the dropdown, enter the value you want to convert, choose whether you are converting from mcg to IU or IU to mcg, and click Convert. The results show the converted value, equivalent in milligrams and grams, plus a detailed breakdown. The comparison chart displays how your selected substance's conversion factor compares to other common vitamins.
Regional Notes
India (IN): Supplement labels in India commonly list vitamin D in IU and vitamin A in mcg. The Recommended Dietary Allowance (RDA) for vitamin D in India is 400-600 IU per day for adults, and for vitamin A it is 600-900 mcg RAE per day depending on age and gender.
United States (US): The FDA requires supplement labels to list vitamin D in mcg (not IU) as of 2020, though many brands still show IU in parentheses. Vitamin A is listed in mcg RAE. The RDA for vitamin D is 600 IU (15 mcg) for adults up to age 70 and 800 IU (20 mcg) for those over 70.
United Kingdom (UK): UK supplement labels typically display vitamin D in micrograms with IU in brackets. Public Health England recommends 10 mcg (400 IU) of vitamin D daily year-round for everyone over the age of one.
Frequently Asked Questions
How do I convert mcg to IU?
To convert mcg to IU, multiply the number of micrograms by the substance-specific conversion factor. For vitamin D, multiply by 40 (1 mcg = 40 IU). For vitamin A (retinol), multiply by 3.33. Each substance has its own IU per mcg ratio because IU measures biological activity, not mass.
How many IU are in 125 mcg of vitamin D?
There are 5,000 IU in 125 mcg of vitamin D (D2 or D3). This is calculated by multiplying 125 mcg by the conversion factor of 40 IU per mcg.
How do I convert IU to mcg?
To convert IU to mcg, divide the number of international units by the substance-specific conversion factor. For vitamin D, divide by 40 to get mcg. For vitamin E natural, divide by 1.49. The formula is: mcg = IU / (IU per mcg factor).
What is the difference between IU and mcg?
IU (International Unit) measures the biological activity or effect of a substance, while mcg (microgram) measures its mass. Two substances with the same weight can have different IU values because they differ in biological potency. This is why conversion factors vary by substance.
Which vitamins use IU measurements?
Vitamins A, D, and E commonly use IU on supplement labels. Vitamin A is measured as retinol activity equivalents, vitamin D as cholecalciferol (D3) or ergocalciferol (D2), and vitamin E as alpha-tocopherol. Vitamin C, B vitamins, and vitamin K are typically measured in mg or mcg, not IU.
Is mcg the same as mg?
No, mcg (microgram) is 1,000 times smaller than mg (milligram). 1 mg = 1,000 mcg. When converting supplement dosages, always check whether the label uses mcg or mg to avoid taking ten times or one-tenth the intended dose.
What is the conversion factor for vitamin E?
Natural vitamin E (RRR-alpha-tocopherol) has a conversion factor of 1.49 IU per mcg. Synthetic vitamin E (all-rac-alpha-tocopherol) has a conversion factor of 1.1 IU per mcg. Always check whether your supplement contains natural or synthetic vitamin E for accurate conversion.
Why do different forms of vitamin A have different IU conversion factors?
Different forms of vitamin A have different biological activities. Retinol (preformed vitamin A) has 3.33 IU per mcg, while beta-carotene (provitamin A) has 1.67 IU per mcg because the body must convert beta-carotene to retinol. Retinyl palmitate has 1.82 IU per mcg and retinyl propionate has 2.78 IU per mcg.