How Much Is 400 IU in Milligrams or Micrograms?

For vitamin D, which is by far the most common reason people encounter a “400 IU” label, 400 IU equals 10 micrograms (mcg). For vitamin E, the same 400 IU translates to roughly 268 milligrams of the natural form or 180 milligrams of the synthetic form. The reason a single number like 400 IU can correspond to wildly different masses is that IU, or International Units, measure biological activity rather than weight, and each vitamin has its own conversion factor. That means there is no universal answer to “how much is 400 IU” until you know which substance you are talking about.

Why There Is No Single Conversion

International Units were invented in the early twentieth century as a way to standardize the potency of biological substances whose chemical identity was not yet pinned down. A batch of vitamin D from cod liver oil might weigh the same as a batch from irradiated yeast yet have very different effects in the body, so scientists pegged the dose to what the substance actually did, not how much it weighed. Even after the molecules were fully characterized, the IU system stuck around because doctors and manufacturers were used to it.

The practical consequence is that each vitamin has its own IU-to-mass ratio, set by international agreement and based on the biological potency of a defined reference compound. Vitamin D’s conversion factor is tiny because the molecule is extraordinarily potent per microgram. Vitamin E’s factor is far larger because you need hundreds of milligrams to achieve the same number of IU. Vitamin A sits somewhere in between. If you swap one vitamin’s conversion factor for another, you can end up off by a factor of a thousand or more, which is why it matters to match the right factor to the right vitamin.

Vitamin D at 400 IU

The conversion for vitamin D is straightforward: one IU equals 0.025 micrograms, regardless of whether the supplement contains vitamin D₂ (ergocalciferol) or vitamin D₃ (cholecalciferol). Multiply that out and 400 IU equals exactly 10 mcg. In milligrams, that is 0.01 mg, a speck so small it would be invisible on a kitchen scale. This is why vitamin D doses are almost always expressed in micrograms or IU rather than milligrams.

400 IU (10 mcg) has been a benchmark dose for decades. It is the amount found in most standard multivitamins and the daily value historically recommended for infants and many adults to prevent deficiency. Many health organizations now suggest higher intakes for adults, often 600 to 2,000 IU (15 to 50 mcg) depending on age and risk factors, but 400 IU remains the dose you will see on the widest range of products. If you pick up a bottle and it says “10 mcg (400 IU)” on the new-style label, those two numbers are the same dose expressed in different units.

Vitamin E at 400 IU

Vitamin E conversions are messier because the molecule comes in multiple forms, and natural and synthetic versions behave differently in the body. Under the older IU system still printed on many bottles, one IU of natural vitamin E (d-alpha-tocopherol) equals 0.67 mg, and one IU of synthetic vitamin E (dl-alpha-tocopherol) equals 0.45 mg. That gives you these round numbers for a 400 IU supplement:

Natural (d-alpha-tocopherol): 400 × 0.67 = about 268 mg
Synthetic (dl-alpha-tocopherol): 400 × 0.45 = about 180 mg

The gap between those two figures is not a labeling quirk. It reflects the fact that synthetic vitamin E is a mixture of eight mirror-image molecular forms called stereoisomers, and the human body does not treat them all equally. Natural vitamin E consists of a single stereoisomer, RRR-alpha-tocopherol, which is the form the body absorbs and retains most efficiently. Synthetic vitamin E contains equal parts of all eight stereoisomers, but only the four so-called 2R forms have meaningful biological activity in humans; the four 2S forms are barely bioavailable at all.¹ Research in cows found that even though the RRR isomer made up only about 20% of the alpha-tocopherol consumed from a synthetic supplement, it accounted for roughly 60% of the alpha-tocopherol that ended up in plasma and milk, because the body almost completely rejected the 2S isomers.² The pattern holds in humans, which is why the IU system assigned synthetic vitamin E a lower mass-per-IU value: you need more of the powder to deliver the same biological punch.

The FDA’s updated labeling rules, phased in starting around 2020, moved vitamin E away from IU altogether and onto milligrams of alpha-tocopherol (mg AT), using a new conversion that accounts for the stereoisomer issue differently. Under the new system, the ratio between natural and synthetic forms shifted, and an old “400 IU” supplement may now read as a different number of mg AT on the new-style Supplement Facts panel. If you are comparing two vitamin E bottles and one uses IU while the other uses mg AT, be aware that the numbers are not directly interchangeable without checking which system each label uses.

Vitamin A at 400 IU

Vitamin A is less commonly sold in a 400 IU dose on its own, but it appears at that level in some multivitamins and pediatric formulations. The conversion depends on the form:

Preformed retinol: 1 IU = 0.3 mcg retinol, so 400 IU = 120 mcg
Beta-carotene from supplements: 1 IU = 0.6 mcg, so 400 IU = 240 mcg
Beta-carotene from food: 1 IU = 1.2 mcg, so 400 IU = 480 mcg

The newer labeling unit for vitamin A is mcg RAE (Retinol Activity Equivalents), which folds the different forms into a single scale based on how efficiently the body converts each one to active retinol. As with vitamin E, the shift away from IU was partly motivated by the fact that IU obscured real differences in how much usable vitamin A you were actually getting. If your multivitamin lists vitamin A in mcg RAE, the number already accounts for the form; if it still lists IU, you need to know whether it contains retinol, beta-carotene, or a blend before the conversion makes sense.

Why Labels Are Shifting Away From IU

The FDA required most food and supplement manufacturers to switch to metric units (mcg for vitamin D, mg AT for vitamin E, mcg RAE for vitamin A) on updated Nutrition Facts and Supplement Facts panels. The deadline for large manufacturers passed in 2020, and smaller companies followed in 2021. The intent was to reduce confusion by putting everything in mass-based units that are consistent across products.

In practice, the transition has created its own brand of confusion. Many supplements now print both the old and new units, and shoppers see something like “Vitamin D₃ 25 mcg (1,000 IU)” without realizing the two numbers represent exactly the same dose. Some consumers, accustomed to thinking in IU, worry that a product labeled “10 mcg” is weaker than one labeled “400 IU.” It is not; those are the same amount of vitamin D. Others try to do mental math between old and new labels for vitamin E and get tripped up by the fact that the conversion factors themselves changed when the labeling standard shifted.

If you are trying to match a doctor’s recommendation given in IU to what a new-format label shows in micrograms or milligrams, the simplest approach is to use the specific vitamin’s conversion factor. For vitamin D, divide the IU by 40 to get micrograms. For vitamin E, the math depends on whether the product is natural or synthetic and whether you are using the old IU definition or the new mg AT standard, so the safest move is to compare the actual mass and form listed on the panel rather than trying to convert in your head.

Quick-Reference Conversions

Because the question usually arises while staring at a supplement bottle, here is a compact reference for the three vitamins most commonly sold in IU:

Vitamin D: 400 IU = 10 mcg = 0.01 mg
Vitamin E (natural): 400 IU = ~268 mg d-alpha-tocopherol
Vitamin E (synthetic): 400 IU = ~180 mg dl-alpha-tocopherol
Vitamin A (as retinol): 400 IU = 120 mcg = 0.12 mg

Notice how dramatically the mass differs across vitamins. 400 IU of vitamin D is ten millionths of a gram; 400 IU of natural vitamin E is more than a quarter of a gram. That contrast is the single biggest reason you cannot just “convert IU to milligrams” without specifying which substance you mean.

When Getting the Conversion Wrong Actually Matters

For a person buying a single over-the-counter vitamin D supplement, mixing up IU and mcg is unlikely to cause harm, because the doses involved are small and there is a wide margin between a typical daily dose and a dangerous one. The risk escalates with fat-soluble vitamins taken at high doses over long periods, or in clinical settings where medications are dosed in units that must be converted to mass for compounding or IV preparation. Hospital analyses have found that a meaningful share of preventable medication errors trace back to incorrect unit conversions, particularly in intensive care settings where drugs may be ordered in one unit system and prepared in another.

Vitamin A is probably the vitamin where a casual conversion mistake poses the most realistic consumer risk. Preformed retinol (the kind in liver, fish oil, and many supplements) can cause toxicity at sustained intakes not enormously higher than the recommended amount, particularly during pregnancy. If someone mistakes milligrams for micrograms and takes a dose that is a thousand times larger than intended, the consequences are real. Vitamin D toxicity is possible but requires very large doses sustained over weeks. Vitamin E at supplemental doses up to about 400 IU per day has generally been considered safe for most people, though some large trials raised questions about risks at that level over many years.

Natural Versus Synthetic and What That Means for Your Dose

The natural-versus-synthetic distinction matters most for vitamin E, though it surfaces with other nutrients too. As noted above, synthetic (dl or “all-rac”) alpha-tocopherol delivers only half its weight as the RRR isomer the body prefers, with the 2S isomers contributing almost nothing to blood levels or tissue stores.² Based on this, one gram of the synthetic form has been estimated to be equivalent to roughly half a gram of the natural form in terms of what actually reaches your tissues.²

The old IU system tried to account for this by assigning different mass-per-IU factors to natural and synthetic forms, so that “400 IU” would theoretically deliver the same biological activity regardless of source. In practice, the equivalence was approximate. Some researchers have argued that the adjustment factor the IU system used underestimated the advantage of natural vitamin E, because the body discriminates against 2S isomers even more strongly than the standard ratio assumed.¹ The newer mg AT labeling system tries to handle this differently, but the underlying biology remains the same: if you care about how much vitamin E actually ends up functional in your body, the source matters, and a straight mass comparison between a natural and a synthetic product will mislead you.

You can usually tell which form a supplement contains by scanning the ingredient list. “d-alpha-tocopherol” or “RRR-alpha-tocopherol” indicates the natural single-isomer form. “dl-alpha-tocopherol” or “all-rac-alpha-tocopherol” indicates the synthetic eight-isomer mix. Some products blend the two, which complicates conversion further. If a supplement simply says “vitamin E” without specifying, it is most often the synthetic form, because it is cheaper to manufacture.

Does the Amount on the Label Stay Accurate Over Time

Manufacturers typically add overages, putting slightly more of a vitamin into a product than the label claims, to account for degradation during the product’s shelf life. Even so, vitamins A and E lose potency gradually during storage, particularly at higher temperatures. A study of powdered enteral formulas found that vitamin A and vitamin E content decreased progressively as storage temperature and time increased, with degradation following a predictable pattern and proceeding faster at elevated temperatures.³ At a moderate storage temperature of about 25°C and 60% humidity, the vitamins held up reasonably well, but higher heat accelerated the decline.

For a typical consumer, this means that a bottle of vitamin D or E capsules stored in a cool, dry cabinet will remain close to label potency through its printed expiration date. A bottle left in a hot car, a steamy bathroom, or a kitchen cabinet next to the stove may deliver less than what the label promises. The dose difference from normal degradation over a shelf life is small enough that it rarely matters clinically for standard supplement doses, but it is another reason to treat the number on the label as an approximation rather than an exact figure, especially as a product ages.

Other Substances Measured in IU

Vitamins A, D, and E are the supplements most commonly encountered in IU, but the system applies to other biological substances as well. Insulin, for instance, is dosed in International Units, though the conversion to mass is irrelevant for most patients because insulin is prescribed and delivered strictly in units, and syringes and pens are calibrated accordingly. Some enzymes, hormones used in fertility treatment, and certain vaccines also use IU or related unit systems. In each case, the IU-to-mass conversion is specific to that substance and defined against an international reference standard held by agencies like the World Health Organization.

For everyday supplement shoppers, the important takeaway is simpler: IU is not a universal mass unit, and the only way to convert it to milligrams or micrograms is to know which vitamin or substance you are dealing with. Once you know that, the math is multiplication by a single fixed number. For the most common case, vitamin D at 400 IU, that number is 10 micrograms, and both values mean exactly the same thing.