3D Vitamin D Capsule Model


Vitamin D, a fat-soluble vitamin naturally produced in the skin in response to sunlight, is also one of the most purchased over the counter (OTC) supplements. In 2017 alone, supplemental Vitamin D accounted for  $937 million in sales!

A common source of confusion is how the strength of Vitamin D is listed on the supplement label. It's not uncommon to see it listed in:

  • Milligrams
  • Micrograms
  • International Units

The International Unit (IU), which refers to the "biological activity" of a drug or substance, is the most common way in which the strength of Vitamin D is listed.

Here is a vitamin D product manufactured by Nature Made that is labeled as 1000 International Units:

Nature Made VItamin D 1000Unit On Front

International Units are used to standardize different forms of the same substance, making them easier to compare. For example:

  • Vitamin A: One International Unit (IU) equals 0.3 mcg of retinol or 0.6 mcg of beta-carotene.
  • Vitamin E: One International Unit (IU) equals 0.67 mg of d-alpha-tocopherol or 0.9 mg of dl-alpha-tocopherol
  • Vitamin D: One International Unit (IU) equals 0.025 mcg of cholecalciferol (Vitamin D3) or of ergocalciferol (Vitamin D2)

The Move Away From International Units

Starting in 2021, the International Unit will no longer be the only strength listed on vitamin D supplements. In fact, it may not be listed at all.

New dietary supplement labeling requirements from the FDA (Food and Drug Administration) will go into full effect by January 2021. Among other things, they will require the strength of vitamin A, D and E to be listed as a metric measure (e.g., milligrams or micrograms).

Per the Food Labeling: Revision of the Nutrition and Supplement Facts Labels:

"In addition to changing some DVs [Daily Values], the proposed rule would change the units used to declare vitamins A, E, and D from ‘‘international units,’’ or ‘‘I.U.’’ to a metric measure, milligrams or micrograms, and also would include the absolute amounts in milligrams or micrograms of vitamins and minerals, in addition to the % DV, on the label."

Many manufacturers have already started following these new requirements, by listing both the IU strength and the metric strength on their labels. Here is one example:

Now Vitamin D In Units And Metric

Hopefully, manufacturers will list both the International Units and the metric strength to assist individuals in finding the appropriate product. However, there certainly may be situations where you need to convert between one or the other.

Fortunately, the conversion is easy:

  • 1 mcg equals 40 IU

Similarly, going from IU to mcg:

  • 1 IU equals 0.025 mcg

The most common strengths you will see are:

  • Vitamin D 400 IU = 10 mcg
  • Vitamin D 1,000 IU = 25 mcg
  • Vitamin D 2,000 IU= 50 mcg
  • Vitamin D 5,000 IU = 125 mcg
  • Vitamin D 10,000 IU = 250 mcg
  • Vitamin D 50,000 IU = 1,250 mcg (1.25 mg)

So, the specific answer to your question, "400 units of Vitamin D is how many micrograms?", the answer is 10 mcg.

If you need assistance converting between IU and mcg, you can use our Vitamin D Conversion Tool that will convert between all 3 forms (IU, mcg, mg).

Vitamin D Conversion Tool

Type a value in any of the fields to convert between Vitamin D measurements:


*Note: This tool rounds up 1 IU to 0.03 mcg. All other conversions are precise.

It is important to note that both Vitamin D2 and Vitamin D3 use the same conversion.

  • For example, 1,000 units of Vitamin D2 and Vitamin D3 both equal 25 mcg.