If you searched for a “mcg to IU converter” or “IU to mcg converter,” you are almost certainly looking at a vitamin label and trying to reconcile two different numbers for the same dose. The short answer: only vitamins A, D, and E use International Units (IU), the conversion factor is different for each one, and this page gives you a verified calculator plus the exact factors and worked examples to do it by hand.
Medically reviewed for unit-conversion accuracy. Conversion factors on this page match the U.S. FDA labeling guidance and the NIH Office of Dietary Supplements Health Professional fact sheets.
Vitamin Unit Converter: IU ↔ mcg / mg
Convert between International Units (IU) and metric mass for the fat-soluble vitamins that actually use IU:
- Choose the direction (mass → IU, or IU → mass)
- Enter the value
- Select the vitamin and form
- Click “Convert”
Note: Only vitamins A, D, and E use IU. Vitamin C, B12, folate, vitamin K and the other B vitamins are measured by mass only (mg or mcg) and have no IU conversion. Vitamin E is entered in milligrams (mg); vitamins A and D in micrograms (mcg). The Vitamin A option uses preformed retinol (mcg RAE); beta-carotene uses a different basis, explained in the reference table below.
Why the conversion is not a single number
An International Unit is not a unit of mass. It is a unit of biological activity, defined separately for each substance. That is why "1 IU" weighs a different amount for vitamin D than it does for vitamin A or vitamin E. There is no universal "mcg to IU" factor. You must know which vitamin you have, and for vitamins A and E you also need to know the chemical form.
The other thing people get wrong is assuming every vitamin uses IU. Most do not. Water-soluble vitamins like vitamin C, B12, and folate, and even fat-soluble vitamin K, are measured purely by mass. Putting them in an IU converter is a category error, so they are deliberately absent from the tool above. If you want a dedicated single-vitamin tool, the vitamin D unit converter covers the most common case on its own page.
Full IU conversion reference table
These are the only conversion factors you need. Each row shows the mass that equals one IU, and the IU that equals one microgram or milligram of that form.
| Vitamin / Form | 1 IU = | 1 mcg or mg = | Notes |
|---|---|---|---|
| Vitamin D (D2 ergocalciferol or D3 cholecalciferol) | 0.025 mcg | 1 mcg = 40 IU | D2 and D3 use the identical IU factor. Divide IU by 40 to get mcg. |
| Vitamin A, retinol (RAE) | 0.3 mcg | 1 mcg = 3.333 IU | Applies to preformed retinol. New labels use mcg RAE. |
| Vitamin A, beta-carotene (dietary, IU convention) | 0.6 mcg | 1 mcg = 1.667 IU | The historical IU basis for beta-carotene. RAE weighting differs (see below). |
| Vitamin E, natural d-alpha-tocopherol (RRR) | 0.67 mg | 1 mg = 1.49 IU | Entered in mg, not mcg. The "d-" prefix means natural. |
| Vitamin E, synthetic dl-alpha-tocopherol (all-rac) | 0.45 mg | 1 mg = 2.22 IU | Synthetic is about half as potent by weight. The "dl-" prefix means synthetic. |
| Vitamin C, B12, folate, vitamin K, and other B vitamins | No IU | No IU | Mass only (mg or mcg). There is no valid IU conversion for these. |
You can verify the factors against the NIH Office of Dietary Supplements vitamin D fact sheet and the NIH ODS vitamin E fact sheet, which lists the natural versus synthetic milligram-to-IU figures used above. For the mass-only vitamins that never use IU, our vitamin B12 dosage guide and the roundups of the best vitamin B12 supplements and best vitamin K2 supplements explain why they stay in milligrams and micrograms.
Why new bottles say mcg or mg instead of IU
If your older bottle said "400 IU" and your new one says "10 mcg," nothing shrank. The label rules changed. In the final rule "Food Labeling: Revision of the Nutrition and Supplement Facts Labels" (81 FR 33742, published in the Federal Register on May 27, 2016), the FDA updated the Nutrition and Supplement Facts panels to align with the National Academy of Medicine's updated Dietary Reference Intakes. Part of that overhaul replaced International Units with metric weights for the fat-soluble vitamins and switched folate and niacin to equivalents.
Here is what changed, unit by unit:
- Vitamin D is now declared in micrograms (mcg). The Daily Value is 20 mcg, which equals 800 IU because 1 mcg = 40 IU. The older Daily Value of 400 IU corresponds to just 10 mcg, so do not equate the legacy 400 IU figure with today's 20 mcg DV. Manufacturers may voluntarily add the IU amount in parentheses next to the mcg value during the transition.
- Vitamin E is now milligrams of alpha-tocopherol, with a Daily Value of 15 mg. Natural ("d-" / RRR) and synthetic ("dl-" / all-rac) forms differ: 1 mg alpha-tocopherol equals 1.49 IU natural or 2.22 IU synthetic, so synthetic is roughly half as potent by weight. IU may be shown voluntarily in parentheses.
- Vitamin A is now mcg RAE (Retinol Activity Equivalents), with a Daily Value of 900 mcg RAE. On the RAE basis, 1 mcg RAE = 1 mcg retinol = 2 mcg supplemental beta-carotene = 12 mcg dietary beta-carotene. IU may be shown voluntarily in parentheses.
- Vitamin C keeps its milligram unit. The Daily Value was revised to 90 mg.
- Folate is now mcg DFE (Dietary Folate Equivalents), with a Daily Value of 400 mcg DFE. Synthetic folic acid is more bioavailable, so 1 mcg folic acid = 1.7 mcg DFE, and when folic acid is present the label must also show its amount in mcg in parentheses.
The compliance dates were staggered. Manufacturers with $10 million or more in annual food sales had to comply by January 1, 2020. Manufacturers under $10 million got a one-year extension to January 1, 2021. Certain single-ingredient sugars such as honey and maple syrup, and some cranberry products, had until July 1, 2021. These dates were themselves extensions from the original 2018 deadline. You can read the official timeline at the FDA's Changes to the Nutrition Facts Label page, and the full rule in the Federal Register.
Reading an old IU bottle next to a new mcg/mg bottle
The lower-looking metric numbers on new labels usually represent the same dose, not a reduced one. Convert before you assume the formula changed.
- Vitamin D is the clean case: 1000 IU = 25 mcg, 2000 IU = 50 mcg. Just divide IU by 40.
- Vitamin A cannot be cleanly converted from old IU without knowing the source. Old IU figures, which mixed retinol and carotenoids, tend to overstate the "active" amount compared with the new RAE basis.
- Vitamin E is the trickiest. The same "400 IU" is about 268 mg if natural but only about 180 mg if synthetic. To recover IU from a milligram figure, multiply by 1.49 (natural) or 2.22 (synthetic).
For the deeper accuracy details behind these factors, the FDA guidance on converting units of measure for folate, niacin, and vitamins A, D, and E is the primary industry reference.
Vitamin D: worked examples (the most common search)
Vitamin D is by far the most searched conversion because almost every supplement bottle now lists both numbers, or only one. The rule never changes: multiply mcg by 40 to get IU, and divide IU by 40 to get mcg. The form does not matter here, because D2 (ergocalciferol) and D3 (cholecalciferol) share the identical IU factor.
- 25 mcg = 1000 IU. The current Daily Value sits just below this, at 20 mcg (800 IU).
- 1000 IU = 25 mcg. The most common low-dose softgel.
- 2000 IU = 50 mcg. A frequent maintenance dose.
- 5000 IU D3 = 125 mcg. A common high-dose supplement.
- 10,000 IU = 250 mcg. A high therapeutic dose that should be used only under guidance.
For dose-specific guidance and product picks, see our roundup of the best vitamin D supplements and the dedicated vitamin D unit converter.
Vitamin A: retinol, RAE, and the old IU problem
Vitamin A is where most conversion errors happen, because the IU system and the modern RAE system measure different things. In the IU convention used on older labels, 1 IU of retinol equals 0.3 mcg, so 900 mcg RAE of retinol equals 3000 IU, and 5000 IU of retinol equals 1500 mcg. Beta-carotene used a separate IU basis (1 IU = 0.6 mcg), which is why a single "IU" number on an old multivitamin could blend two very different sources.
The RAE system replaced that ambiguity. It weights sources by how much usable retinol the body actually gets: 1 mcg RAE equals 1 mcg retinol, 2 mcg supplemental beta-carotene, or 12 mcg dietary beta-carotene. That is why you cannot back-convert an old "IU" figure into mcg RAE without knowing the source. Our roundup of the best vitamin A supplements walks through retinol versus beta-carotene forms in detail.
Vitamin E: why milligrams, not micrograms
Vitamin E is the one fat-soluble vitamin entered in milligrams rather than micrograms, and the natural-versus-synthetic distinction matters more than for any other vitamin. Natural d-alpha-tocopherol (RRR) gives 1.49 IU per milligram; synthetic dl-alpha-tocopherol (all-rac) gives only 2.22 IU per milligram, which means it takes more milligrams of natural E to reach the same IU figure. Working it the other way, 15 mg of natural E equals about 22.4 IU.
Read the label prefix carefully: "d-alpha" is natural, "dl-alpha" is synthetic. If your bottle only lists IU, you still need to know which form it is before you can convert to milligrams accurately. Our roundup of the best vitamin E supplements covers natural-versus-synthetic form selection and dosing.
Frequently asked questions
How many IU are in 1 mcg of vitamin D?
1 mcg of vitamin D equals 40 IU, because 1 IU is defined as 0.025 mcg. To convert micrograms to IU, multiply by 40; to convert IU to micrograms, divide by 40. So 25 mcg equals 1000 IU and 1000 IU equals 25 mcg. This factor is identical for D2 (ergocalciferol) and D3 (cholecalciferol).
How do I convert 5000 IU of vitamin D3 to mcg?
Divide 5000 by 40, which gives 125 mcg. A 5000 IU vitamin D3 softgel therefore contains 125 mcg of cholecalciferol. The same arithmetic works for any dose: 2000 IU is 50 mcg, and 10,000 IU is 250 mcg.
Why is vitamin E entered in mg instead of mcg?
Vitamin E doses are large enough that milligrams are the practical unit, and modern labels declare it as milligrams of alpha-tocopherol with a Daily Value of 15 mg. The IU-to-mass factor also depends on form: natural d-alpha-tocopherol gives 1.49 IU per mg, while synthetic dl-alpha-tocopherol gives 2.22 IU per mg. So 15 mg of natural E equals roughly 22.4 IU.
Does vitamin C use International Units?
No. Vitamin C is measured by mass only, in milligrams, with a Daily Value of 90 mg. The same is true for vitamin B12, folate, vitamin K, and the other B vitamins. Only vitamins A, D, and E ever used IU, so there is no valid mcg-to-IU conversion for vitamin C.
How do I convert vitamin A from IU to mcg?
For preformed retinol, 1 IU equals 0.3 mcg, so multiply the IU figure by 0.3. That makes 5000 IU of retinol equal to 1500 mcg, and 900 mcg RAE of retinol equal to 3000 IU. Beta-carotene used a different IU basis (1 IU = 0.6 mcg), which is why an old "IU" figure that blended sources cannot be cleanly back-converted to mcg RAE without knowing the source.
A note on safe dosing
Converting units does not tell you whether a dose is safe. Vitamins A, D, and E are fat-soluble and accumulate in the body, so they have tolerable upper intake levels: 100 mcg (4000 IU) per day for vitamin D, 3000 mcg RAE for preformed vitamin A, and 1000 mg for supplemental vitamin E in adults. Converting an old IU label to mcg or mg can reveal that a "high-potency" product sits near or above these limits. If you take prescription doses, multiple supplements, or any medication, check for interactions with our drug and supplement interaction checker and see how we review supplements before changing a regimen.
This converter and article are for general educational purposes and are not medical advice. Conversion factors follow U.S. FDA labeling guidance and NIH Office of Dietary Supplements fact sheets. Talk to your doctor or pharmacist before starting, stopping, or changing the dose of any supplement, especially the fat-soluble vitamins A, D, E, and K.
