mcg to IU Converter

Created by Łucja Zaborowska, MD, PhD candidate
Reviewed by Anna Szczepanek, PhD, Rijk de Wet and Aleksandra Zając, MD
Based on research by
Hewitt W. Theory and application of Microbiological Assay (December 1988)See 4 more sources
National Institutes of Health, Office of Dietary Supplements Dietary Supplement Ingredient Database - Unit Conversions (May 2016)WHO National Institute for Biological Standards and Control WHO International Standard OXYTOCIN 4th International Standard April 2013World Health Organisation WHO International Biological Reference Preparations (January 2016)WHO Expert Committee on Biological Standardization WHO International Collaborative Study of the Proposed 4th International Standard for Prolactin, Human (October 2016)
Last updated: May 22, 2023

Welcome to the mcg to IU converter — a painless way to turn the number of international units (IU) to either mcg (μg, micrograms), mg (milligrams), or g (grams) for a given substance.

Read on to discover some crucial facts about IU and learn how to convert IU to mg or other metric units.

💡 Just to remind you — all of our calculators work both ways! That's why it's just as easy to convert mcg to IU as IU to mg.

We try our best to make our Omni Calculators as precise and reliable as possible. However, this tool can never replace a professional doctor's assessment. If any health condition bothers you, consult a physician.

What is IU?

IU, the international unit, is a measure of the biological activity of a given substance. The IU defines the effect, not the weight of a medium.

The IU value is established by an international agreement. It is different for every drug, blood product, vitamin, vaccine, or hormone. 💉

Why do we use IU?

The international unit (IU) can usually be spotted on packages of vitamin A, vitamin D, hCG, or insulin.

Sometimes it is easier to express the effect of a substance needed than the weight of it. For instance., suppose we know the required activity. In that case, if two similar substances have exactly the same weight but different activities, we can still easily calculate how much of them we really need to fulfill our requirements.

💡 We offer a range of different vitamin calculations!
Start your journey with the vitamin A calculator or check whether you spend enough time in the sun using the vitamin D calculator.

How does converting from mcg to IU work?

Since International Units and metric weight units describe two entirely different qualities, we can't convert all the substances in the same way. We need to know the exact value of IU per mcg that should be applied in a given situation.

Our mcg to IU converter comprises an extensive list of different compounds that can be translated into metric units and vice versa. They are all sorted in alphabetical order.


How do I convert mcg to IU for vitamin A?

To perform the conversion from mcg to IU for vitamin A:

  1. Find the conversion factor for vitamin A (retinol): 3.33 IU per mcg.

  2. Make use of a simple formula:

    Amount in IU = Amount in mcg × 3.33 IU per mcg

  3. That's it! You're good to go. 🎉

Also, you can turn IU to mg by choosing the equation:

Amount of mg = Amount of IU × 0.0003.

How many IU is 500 mcg?

The conversion from IU into mcg will depend on the specific substance. For 500 mcg:

  • There are 20,000 IU of vitamin D. It applies to both vitamin D2 and D3 (Cholecalciferol);
  • For retinyl palmitate (a form of Vitamin A), there are 909 IU; and
  • For beta-carotene (another form of Vitamin A), there are 833.3 IU.

Other substances, such as Vitamin B12, calcitriol, biotin, and Vitamin K, are not expressed in IU.

How many mcg are in 1500 IU?

The number of mcg in 1500 IU will again depend on the substance to convert. For example:

  • There are 37.5 mcg in 1500 IU of vitamin D (D2 or D3).
  • For vitamin A, it depends on its form. For example, in 1500 IU of vitamin A:
    • There are 825.1 mcg of retinyl palmitate;
    • 900 mcg of beta-carotene;
    • 450 mcg of retinol;
    • 538.5 mcg of retinyl propionate; and
    • 5357 mcg of mixed provitamin carotenoids.

How many IU are in 125 mcg of vitamin D?

There are 5000 IU in 125 mcg of vitamin D. That's the result of multiplying the number of IU in an mcg of vitamin D (40 IU per mcg) by the number of micrograms in this example (125):

Number of IU = Amount in mcg × 40 IU per mcg

Number of IU = 125 × 40 IU per mcg = 5000 IU

Łucja Zaborowska, MD, PhD candidate
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