International Units to micrograms and milligrams, substance by substance — and an honest list of the substances where that conversion does not exist.
Calculator
Common labels:
1000 IU of vitamin d (cholecalciferol or ergocalciferol)
25.000 mcg
25.000 micrograms · 0.02500 milligrams · The one clean case: 40 IU is exactly 1 µg, and both D2 and D3 use the same factor.
The same 1000 IU, for every substance that has a factor
One IU count, several different masses
Substance
Micrograms per IU
Mass
Vitamin D (cholecalciferol or ergocalciferol)
0.025
25.000 mcg
Vitamin A, as retinol
0.3
300.0 mcg
Vitamin A, as beta-carotene
0.6
600.0 mcg
Vitamin E, natural (RRR-alpha-tocopherol)
670
670.0 mg
Vitamin E, synthetic (all-rac-alpha-tocopherol)
450
450.0 mg
Human insulin
34.7
34.700 mg
An International Unit measures biological activity, not mass. Each factor above was fixed by an international assay for one specific substance, so the column of masses is not a conversion table so much as a list of unrelated definitions that happen to share a name.
Two vitamins where the form changes the answer
Vitamin A: retinol against beta-carotene2.00×0.3 µg per IU against 0.6
Vitamin E: natural against synthetic1.489×0.67 mg per IU against 0.45
The IU did not change in either row — the substance did. Beta-carotene needs twice the mass to deliver one IU of vitamin A activity because the body converts it less efficiently, and the synthetic vitamin E isomer mixture needs about a third less mass than the natural one for the same IU. A converter that offers a single number per vitamin has quietly picked a form on your behalf, and labels do not always say which they used: look for “d-alpha” against “dl-alpha” on vitamin E.
Where an IU does not exist at all
Substances this converter refuses, and why
Substance
Why there is no factor
Heparin
Heparin is standardised by an anticoagulant activity assay, and the activity per milligram varies between preparations. There is no mass equivalent to convert to.
Vitamin C
Vitamin C is not measured in International Units. It is specified by mass, and always has been.
Vitamin B12
Vitamin B12 is not measured in International Units. Labels give micrograms.
Folate / folic acid
Folate is not measured in International Units. It is specified in micrograms, and increasingly in dietary folate equivalents, which are also a mass.
These are listed rather than left out because their absence is a fact about the units, not a gap in this table. Heparin is dosed in IU precisely because its mass would not tell you what it does; vitamin C, B12 and folate have never been expressed in IU at all, so a figure returned for them would be invented.
Micrograms, milligrams and grams
The metric ladder — every step exactly a thousand
Unit
Value
Note
nanogram (ng)
1,000,000.0
A thousandth of a microgram. Met in hormone assays rather than on labels.
microgram (mcg (µg))
1,000.0
Written mcg on labels because µ is lost in handwriting and in plain text.
milligram (mg)
1.000
A thousand micrograms. The commonest unit on a tablet.
gram (g)
0.00100
A thousand milligrams.
Back to IU, as the substance selected above40,000.0 IUVitamin D (cholecalciferol or ergocalciferol)
One thousand micrograms to the milligram. The most consequential single confusion on a label. The spelling mcg exists because the Greek µ is lost in handwriting and in plain text, and because “µg” misread as “mg” is a thousandfold error rather than a typo. If a label and a note disagree about which one is meant, that is worth resolving before doing any arithmetic on it.
What this converter covers
Vitamin A, D, E and insulin with their published factors, both forms of A and both isomers of E, the metric ladder, and four substances that refuse.
Vitamin D, where 40 IU is exactly one microgram
Vitamin A as retinol and as beta-carotene — the same IU, twice the mass
Vitamin E, natural against synthetic alpha-tocopherol
Human insulin, at 28.8 IU per milligram
Heparin, vitamin C, B12 and folate, listed with the reason no conversion exists
Activity, not mass Form-dependent mcg · mg · g Both directions
Free, no signup — exact by definition, not an estimate.
Updated 8 September 2026
At a glance
Formula shown
mass in µg = IU × the factor for that specific substance and form
Scenario support
1000 IU vitamin D is 25 µg · 400 IU vitamin E is 268 mg natural or 180 mg synthetic
Educational estimate
Planning support from the values you enter — not professional advice.
An IU is not an amount
“How many mg is 1000 IU” is one of the most-asked conversion questions there is, and it cannot be answered as asked. The unit is missing half its definition.
An International Unit measures biological activity, not mass. It was created for substances that could be measured by what they did long before they could be purified and weighed — insulin, heparin, vitamins — and each one’s IU was fixed separately, by assay, against a physical reference preparation held by an international standards body.
So there is no general factor. There is a list, and every entry on it was set independently:
What one International Unit is, substance by substance
Substance
One IU is
So 1000 IU is
Vitamin D
0.025 µg
25 µg
Vitamin A, retinol
0.3 µg
300 µg
Vitamin E, natural
0.67 mg
670 mg
Human insulin
34.7 µg
34.7 mg
Look at the range: the same 1000 IU is 25 micrograms of one substance and 670 milligrams of another — a factor of nearly thirty thousand. The IU is doing no work at all across that table. It is four unrelated definitions sharing a name.
Which is why the calculator above will not convert without a substance, and why the number it gives is only valid for the substance you named.
The same IU, two masses
Naming the vitamin turns out not to be enough for two of them, and this is the part most converters get wrong by omission.
Vitamin A has activity from retinol and from beta-carotene, and the body converts beta-carotene less efficiently. So one IU is 0.3 µg of retinol and 0.6 µg of beta-carotene — exactly twice the mass for the same unit.
Vitamin E splits differently. The natural form, RRR-alpha-tocopherol and usually labelled “d-alpha”, is 0.67 mg per IU. The synthetic mixture, all-rac-alpha-tocopherol or “dl-alpha”, is 0.45 mg per IU. Same IU count, about a third less mass, because only some of the synthetic isomers are biologically active.
A 400 IU vitamin E capsule therefore contains 268 mg or 180 mg depending on which form it holds, and both are correctly labelled 400 IU. A converter that returns one number for “vitamin E” has chosen a form silently, and has a 50 per cent error available to it.
The tell is on the bottle. “d-alpha-tocopherol” is natural; “dl-alpha-tocopheryl acetate” is synthetic. If the label gives both an IU figure and a milligram figure, dividing one by the other tells you which form it is, without needing to read the chemistry.
Where no factor exists
Four common searches lead to a conversion that does not exist, and it is worth being specific about the two different reasons.
Heparin has an IU, and it has no mass equivalent. Its potency is set by an anticoagulant activity assay, and the activity per milligram genuinely varies between preparations — which is precisely why it is dosed in units. The mass would not tell a clinician what the dose does. This is the IU working as intended rather than a gap in the tables.
Vitamin C, vitamin B12 and folate are the opposite case: they have no IU at all, and never have. They are specified by mass, in milligrams or micrograms, everywhere. A search for “IU to mg vitamin C” is asking to convert from a unit that was never used for it.
Both are listed in the calculator with the reason attached, rather than being left out of the dropdown. An omission looks like an oversight; a stated refusal is information. If a bottle of vitamin C really does say IU, that is a labelling problem worth querying rather than an arithmetic one.
mcg, mg, and one letter
Under the IU question sits a simpler and more consequential one: micrograms and milligrams are a thousand apart and look almost identical.
The proper symbol is µg. In handwriting the µ can pass for an m; in plain text and on older systems it is dropped or mangled entirely. So the spelling mcg exists specifically to be unmistakable, and medication-safety bodies recommend it for exactly that reason.
The gap is not subtle. A microgram of vitamin D is 40 IU; a milligram is 40,000. Reading one as the other is not a rounding error but a thousandfold one, and it runs in whichever direction the misreading happens to go.
Two habits help. Write mcg rather than µg wherever the text might be copied or rewritten. And sanity-check the magnitude against what a tablet plausibly holds: supplement doses in micrograms are typically in the hundreds or thousands, and in milligrams typically in the tens or hundreds. A figure sitting far outside those bands is usually a prefix that has slipped rather than an unusual product.
Why labels moved away from IU
If all this sounds like a unit that causes more trouble than it solves, regulators agree, and labels have been moving for years.
Supplement and food labelling in a number of jurisdictions now requires vitamins A, D and E to be declared in micrograms or milligrams, with the IU figure optional alongside. Vitamin A moved further, to retinol activity equivalents, which build the form-dependence into the number instead of leaving it to a footnote.
The transition is why so many bottles carry both. A vitamin D label reading “25 mcg (1000 IU)” is stating the same quantity twice, and the pair is a useful check: if the two figures do not sit at 40 IU per microgram, one of them is wrong.
The IU survives where it is still doing real work — insulin, heparin, vaccines, biological products generally — because for those there is no mass that would describe the potency. That is the distinction worth carrying away: an IU is indispensable when activity cannot be inferred from mass, and a nuisance when it can.
Related calculators
Other concentration, mass and clinical-unit tools:
mg/dL to mmol/LLab results between US and SI units — the factor is the molar mass over ten, so it differs for every test, and BUN is not urea.
Moles to GramsGrams, moles, molarity, molality and normality — asking for the molar mass and the density, because neither conversion exists without them.
PPMppm to percent, mg/L and µg/m³ — asking which liquid or which gas, because without that the conversion has no answer.
WeightKilograms, pounds, ounces, stone and tonnes, listing the short, long and metric ton separately — three masses, one word.
Alcohol UnitsAny drink into grams of pure alcohol and into five national standards at once — because a UK unit and a US standard drink differ by 77%.
Scientific NotationScientific, engineering and decimal forms with significant figures counted — and ambiguous inputs flagged rather than silently resolved.
The first two explain what an International Unit is and who assigns one; the third is where the vitamin A and vitamin E form factors come from; and the fourth is why this page writes “mcg” out in full.
WHO Expert Committee on Biological StandardizationWorld Health Organization · verified 2026-09-08 · That an International Unit is established substance by substance against a WHO International Standard by biological assay, rather than derived from any mass or any general definition — which is why the factors on this page are a list rather than a formula
WHO International Standards and Reference ReagentsNational Institute for Biological Standards and Control · verified 2026-09-08 · The custodianship of the physical reference preparations that define International Units for biological substances such as heparin and insulin, and the potency assays through which an IU is assigned
Dietary reference valuesEuropean Food Safety Authority · verified 2026-09-08 · The conversion factors between retinol and beta-carotene for vitamin A activity, and between the natural and synthetic alpha-tocopherol forms of vitamin E — the reason one IU maps to two different masses for the same vitamin
List of error-prone abbreviations, symbols and dose designationsInstitute for Safe Medication Practices · verified 2026-09-08 · That the microgram symbol and the abbreviation for International Unit are both documented sources of medication error, which is why "mcg" is written out and why the thousandfold gap between micrograms and milligrams is called out on this page
Conversion note
This is a unit converter and nothing else. It does not recommend, calculate, check or adjust any dose, for anyone, in any circumstance, and no number it produces should be used to decide what to take or give. Converting a quantity between units says nothing about whether that quantity is appropriate: appropriate intake depends on age, body size, pregnancy, kidney and liver function, other medicines, existing conditions and the specific preparation, and those are matters for a doctor or pharmacist with access to the individual. The factors here are the published conversion factors for the substances named and do not apply to any other substance, including chemically related ones — insulin analogues, other tocopherols and other retinoids each have their own. Product labels differ in which form they contain and do not always say; where a label and this page disagree, the label describes what is actually in the bottle. Some vitamins are harmful in excess, particularly the fat-soluble ones, and an arithmetic error in either direction matters. If a prescription, a label and a calculation do not agree, stop and ask a pharmacist rather than resolving it yourself.
Published the IU to mg Converter: International Units to micrograms and milligrams for vitamin D, both forms of vitamin A, both isomers of vitamin E and human insulin, plus the nanogram-to-gram metric ladder.
States the thing that makes the question unanswerable as usually asked: an International Unit measures biological ACTIVITY, not mass, and each substance's factor was fixed separately by assay against an international reference preparation.
Shows the range that proves it. The same 1000 IU is 25 micrograms of vitamin D and 670 milligrams of natural vitamin E -- a factor of nearly thirty thousand, with the IU doing no work at all across the table.
Splits the two vitamins whose factor depends on the FORM: one IU of vitamin A is 0.3 micrograms as retinol and 0.6 as beta-carotene, exactly twice; one IU of vitamin E is 0.67 mg natural and 0.45 mg synthetic, about a third less.
Lists heparin, vitamin C, vitamin B12 and folate in the substance selector with the reason no conversion exists, rather than omitting them -- heparin because its potency varies by preparation and has no mass equivalent, the vitamins because they were never expressed in IU at all.
Calls out the microgram-milligram hazard directly, including why the spelling 'mcg' exists, and gives a magnitude sanity check for spotting a slipped prefix.
Explains why supplement labels have been moving off IU onto micrograms and milligrams, and that a label carrying both is a free consistency check at 40 IU per microgram for vitamin D.
Carries an explicit disclaimer that nothing on the page is a dose, a dose check or a recommendation, and that converting a quantity says nothing about whether that quantity is appropriate.
Verified by 76 automated cases, asserting that each refusing substance returns null AND carries a stated reason, that beta-carotene is exactly twice retinol for the same IU, that the two vitamin E isomers are 1.489 apart, that every mass step is exactly a thousand, and that no blood-level or dose estimator is exported.
Add this calculator to your site
Responsive embed — and private: nothing your visitors type leaves their browser.