Pick up three vitamin D bottles and you may find one labeled in IU, one in mcg, and one showing both. A magnesium bottle says 400 mg on the front and 133 mg on the back. A fish oil capsule says 1,000 mg in large type and 300 mg in small type. None of these are lies, exactly. They are different units, or different things being measured, and the supplement industry has never settled on one system.
Part of the confusion is recent. When the FDA updated the nutrition and supplement label rules (compliance was phased in through 2020 and 2021), vitamins A, D, and E moved from international units to micrograms and milligrams, and folate moved to a unit called DFE. Many products now print both the old and new units, older bottles and most forum advice still speak IU, and research papers use whichever unit the authors preferred. This guide is the decoder.
The basics: g, mg, mcg
Three weight units cover most of the label, and they differ by a factor of 1,000 each step.
| Unit | Equals | Typical use |
|---|---|---|
| g (gram) | 1,000 mg | Protein, creatine, fiber, fish oil totals |
| mg (milligram) | 1,000 mcg | Vitamin C, magnesium, most herbs |
| mcg (microgram) | 1/1,000 mg | B12, vitamin D, folate, selenium, iodine, chromium |
“mcg,” “µg,” and “ug” all mean microgram. The mix-ups here are the dangerous ones because trace nutrients have narrow ranges. A 1,000 mcg B12 tablet is 1 mg. A 0.5 mg melatonin tablet is 500 mcg. Selenium’s RDA is 55 mcg and its upper limit is 400 mcg; a product mislabeled in mg would deliver a toxic dose. Iodine is dosed at 150 mcg. With these nutrients, the unit is the entire story, so read it before the number.
IU: why one unit means three different things
An international unit is not a weight. It is a measure of biological activity, standardized separately for each vitamin, which is why 1 IU of vitamin D and 1 IU of vitamin E are completely different amounts of material. IU is a leftover from the era before pure vitamins could be weighed, and the FDA label update was an attempt to retire it. Because so much guidance still uses it, you need the conversions.
| Vitamin | Conversion | RDA (adults) | Upper limit (adults) |
|---|---|---|---|
| Vitamin D | 1 mcg = 40 IU; 1 IU = 0.025 mcg | 600 IU (15 mcg); 800 IU (20 mcg) over 70 | 4,000 IU (100 mcg) |
| Vitamin E, natural (d-alpha-tocopherol) | 1 mg = 1.49 IU; 1 IU = 0.67 mg | 15 mg (about 22 IU) | 1,000 mg (about 1,500 IU) from supplements |
| Vitamin E, synthetic (dl-alpha-tocopherol) | 1 mg = 1.1 IU; 1 IU = 0.45 mg | 15 mg (about 33 IU) | 1,000 mg (about 1,100 IU) from supplements |
| Vitamin A, retinol | 1 mcg RAE = 3.33 IU; 1 IU = 0.3 mcg RAE | 900 mcg RAE men, 700 mcg RAE women | 3,000 mcg RAE (10,000 IU) preformed only |
| Vitamin A, beta-carotene in supplements | 1 IU = 0.15 mcg RAE | Counts toward RDA | Upper limit does not apply |
Worked examples that come up constantly:
- A “5,000 IU” vitamin D3 softgel is 125 mcg, which is above the 4,000 IU adult upper limit. It may be appropriate under medical guidance for a documented deficiency, but it is not a maintenance dose.
- A “400 IU” natural vitamin E capsule is about 268 mg, roughly 18 times the RDA. That is well under the upper limit, but it is not a small dose either.
- A “10,000 IU” vitamin A supplement is 3,000 mcg RAE, exactly the upper limit, if it is retinol or retinyl palmitate. If it is beta-carotene, it is 1,500 mcg RAE and the upper limit does not apply. The form changes the safety math entirely.
Vitamin E labels carry one more trap. The “d-” prefix means natural, the “dl-” prefix means synthetic, and natural is absorbed and retained roughly 1.4 times better, which is why the IU conversion differs. Two bottles both saying “400 IU” contain different milligram amounts, and the label has already adjusted for potency. If you compare by mg, the natural one looks smaller but delivers the same activity.
RAE, DFE, NE: what “equivalents” mean
Three nutrients are measured in equivalents because the body converts different forms with different efficiency.
RAE (retinol activity equivalents) exist because carotenoids from plants convert to active vitamin A poorly. Twelve micrograms of beta-carotene from food equals one microgram of retinol; from a supplement, where absorption is better, the ratio is two to one. A label reading “900 mcg RAE (100% as beta-carotene)” tells you the manufacturer has done that conversion for you. Preformed retinol is the form with an upper limit and the form that matters in pregnancy; beta-carotene is neither, though very high doses of beta-carotene supplements carry their own concerns, particularly for smokers.
DFE (dietary folate equivalents) exist because synthetic folic acid is absorbed better than the folate in food. The FDA convention: 1 mcg DFE equals 1 mcg of food folate, 0.6 mcg of folic acid taken with food, or 0.5 mcg of folic acid taken on an empty stomach. In practice, labels multiply folic acid by 1.7. So a supplement containing 400 mcg of folic acid is listed as about 667 mcg DFE, often with the folic acid amount in parentheses. The adult RDA is 400 mcg DFE and the pregnancy target is 600 mcg DFE. The 1,000 mcg upper limit is set for synthetic folic acid itself, not DFE, and exists partly because high folic acid intake can mask a B12 deficiency. Methylfolate products use the same DFE convention on their labels.
NE (niacin equivalents) account for the body making niacin from tryptophan; 1 mg NE equals 1 mg of niacin or 60 mg of tryptophan. The 35 mg upper limit applies to supplemental niacin and nicotinamide, because of the flushing and, at much higher doses, liver effects.
Minerals: elemental vs compound
This is the single most misread number on supplement labels. Minerals are never sold as pure metal; they are bound to something (citrate, oxide, glycinate, carbonate) and the compound weighs much more than the mineral inside it. The Supplement Facts panel is required to list the elemental amount, but front-of-bottle marketing and many online listings show the compound weight.
| Compound | Approximate elemental mineral by weight |
|---|---|
| Magnesium oxide | ~60% |
| Magnesium citrate | ~16% |
| Magnesium glycinate | ~14% |
| Magnesium malate | ~15% |
| Calcium carbonate | ~40% |
| Calcium citrate | ~21% |
| Zinc citrate | ~31% |
| Zinc picolinate | ~21% |
| Zinc gluconate | ~14% |
| Ferrous fumarate | ~33% |
| Ferrous sulfate | ~20% |
| Ferrous gluconate | ~12% |
The wording tells you which number you are looking at. “Magnesium (as magnesium citrate) 200 mg” means 200 mg of elemental magnesium. “Magnesium citrate 1,000 mg” means 1,000 mg of the compound, which is about 160 mg of magnesium. The 350 mg upper limit for supplemental magnesium, the 40 mg limit for zinc, and the 45 mg limit for iron all refer to the elemental amount. We cover why the binding compound also affects absorption in our guides to chelated minerals and magnesium forms.
Electrolyte products sometimes use mEq (milliequivalents), a unit from clinical medicine that counts charge rather than weight. For the common ones: 1 mEq of potassium is 39 mg, 1 mEq of sodium is 23 mg, and 1 mEq of magnesium is about 12 mg. Over-the-counter potassium supplements in the US are capped at 99 mg per serving, which is about 2.5 mEq, a fraction of the 4,700 mg daily adequate intake and a reminder that food is the main source.
Oils, herbs, probiotics, and enzymes
Fish and algae oils. The big number is the total oil; the numbers that matter are EPA and DHA in mg. A standard 1,000 mg fish oil softgel typically contains around 300 mg of combined EPA and DHA. Concentrated products may reach 600-900 mg per softgel. Trial doses for most uses run 1,000-2,000 mg of EPA plus DHA per day, which can be one concentrated capsule or four to six standard ones.
Herbal extracts use two systems. An extract ratio like “10:1” means 10 g of raw herb was used to make 1 g of extract; it says nothing about which compounds were concentrated and does not mean “10 times stronger.” A standardization like “95% curcuminoids,” “5% withanolides,” or “80% silymarin” tells you the guaranteed content of a marker compound, which is more useful for comparing products. “Equivalent to 3,000 mg of dried herb” is derived from the extract ratio and is mostly marketing.
Probiotics are measured in CFU (colony-forming units), usually in billions. Two products with “10 billion CFU” can differ sharply depending on whether that count is guaranteed “at time of manufacture” (it will have declined by the time you take it) or “through the best-by date” (the number you will actually get). Look for the latter.
Digestive enzymes are dosed by activity units (DU, HUT, LU, and others defined by the Food Chemicals Codex) rather than milligrams, because weight does not predict how much starch, protein, or fat the enzyme can break down. Compare products within the same unit only.
What %DV does and does not tell you
Percent Daily Value compares a serving to the FDA’s Daily Value for adults, which is a single reference figure rather than the age- and sex-specific RDA. A general rule: 5% DV or less is a low contribution, 20% or more is high. Some Daily Values are worth memorizing because they anchor the percentages: vitamin C 90 mg, vitamin D 20 mcg (800 IU), magnesium 420 mg, B12 2.4 mcg, folate 400 mcg DFE.
Two limits of %DV. First, a large percentage is not a warning by itself; B12 at “41,667% DV” is 1,000 mcg and is safe because absorption is limited. Second, %DV says nothing about the upper limit. A 5,000 IU vitamin D product shows 625% DV, which sounds enthusiastic rather than alarming, yet it exceeds the tolerable upper level. For that comparison you need our supplement upper limits guide, not the percentage.
A five-step routine for any label
- Read the unit before the number. mcg, mg, IU, DFE, CFU. Decide what you are looking at.
- Convert IU to mcg or mg if you are comparing with a study, an RDA, or an upper limit. Use the table above.
- Confirm elemental for minerals. Look for “as” or check the Supplement Facts panel, not the front label.
- Check the serving size. “Per serving” may mean three capsules; the per-capsule dose is a third of the headline.
- Compare to both the RDA and the upper limit. A dose can be far above the RDA and still safe, or modestly above and worth a conversation with your provider.
Our broader walkthrough of the rest of the label, from proprietary blends to “other ingredients,” is in how to read supplement labels.
Safety notes specific to unit confusion
Most unit mistakes waste money. A few cause harm, and they are predictable:
- Vitamin D. Products mislabeled between IU and mcg have caused documented cases of vitamin D toxicity. If a dose seems unusually high, do the conversion before taking it.
- Selenium and iodine. Both are dosed in mcg with upper limits in the hundreds of mcg. A mg-level dose is dangerous.
- Iron. Accidental iron overdose has historically been a leading cause of poisoning in young children. Keep iron products, including adult multivitamins, out of reach.
- Preformed vitamin A in pregnancy. Stay under 3,000 mcg RAE (10,000 IU) of retinol per day; beta-carotene does not carry the same concern.
- Folic acid above 1,000 mcg. Can mask vitamin B12 deficiency; higher doses belong under medical guidance.
Anyone who is pregnant, nursing, taking medication, or managing a health condition should confirm doses with a provider, and the unit conversion is a good thing to bring to that conversation.
Bottom line
Supplement labels use weight (g, mg, mcg), activity (IU), equivalents (RAE, DFE, NE), counts (CFU), and charge (mEq), often on the same bottle. The four conversions that prevent most mistakes are vitamin D at 40 IU per mcg, vitamin E at roughly 1.5 IU per mg for natural or 1.1 for synthetic, vitamin A at 3.3 IU per mcg RAE for retinol, and the elemental fraction for minerals. Read the unit first, convert before comparing, and check the upper limit rather than trusting a percentage. Done routinely, this takes fifteen seconds and prevents the 10x and 40x errors that actually matter.
This guide is for educational purposes only and is not medical advice. Consult a healthcare provider before starting any supplement, especially if you are pregnant, nursing, taking medication, or managing a health condition.