Units and amounts: frequently asked questions
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What is an IU, and how does it relate to micrograms?
An IU (International Unit) measures a vitamin’s biological potency, not its weight, so the same number of IUs is a different weight in micrograms or milligrams for each vitamin and each form of it.
IU exists because early vitamin preparations varied in purity and strength, and stating potency let manufacturers give a comparable dose regardless. In the UK, vitamin D3, vitamin E and vitamin A are the vitamins most often labelled this way, sometimes alongside a micrograms or milligrams figure; vitamin C, the B vitamins and minerals are given only in milligrams or micrograms. Because the conversion differs by vitamin and by chemical form, always use the specific factor rather than assuming one number fits every vitamin.
See our conversion factors for the figures we use.
How many micrograms is 1,000 IU of vitamin D? How many is 400 IU? How many is 4,000 IU?
For vitamin D3, 40 IU equals 1 microgram, so 1,000 IU is 25 micrograms (µg), 400 IU is 10 micrograms, and 4,000 IU is 100 micrograms.
This factor is fixed by international agreement and is the same for every cholecalciferol (D3) product, whatever the brand or pack size. UK labels increasingly print the microgram figure on its own, imported products often still use IU, and some print both; multiplying or dividing by 40 converts between them either way, since it is the same underlying amount. Vitamin D2 (ergocalciferol) uses the same 40 IU per microgram conversion.
Compare vitamin D3 prices per 1,000 IU, or see how we convert units.
Is mcg the same as µg?
Yes: “mcg” and “µg” both mean microgram, one thousandth of a milligram; “mcg” is simply the version that avoids the Greek letter µ (mu), which does not display correctly on every system or old label printer.
UK and EU labelling rules use the “µg” symbol, so most supplement labels sold here print that; some ingredient databases and US-influenced brands write “mcg” instead. Neither is more or less accurate, and a product labelled “1,000 mcg B12” contains exactly the same amount as one labelled “1,000 µg B12”. Our own price tables print “µg”, so if a label you are reading says “mcg”, read it as the same unit.
See our full glossary of label terms.
How many milligrams are in a gram?
There are 1,000 milligrams (mg) in one gram (g), and 1,000 micrograms (µg) in one milligram, because the metric system scales every unit by a thousand at each step.
This matters on supplement labels because the same amount is sometimes printed in grams and sometimes in milligrams depending on the ingredient and the manufacturer’s house style: “5 g creatine monohydrate” and “5,000 mg creatine monohydrate” are the same serving. Fish oil and other oils are almost always given in milligrams, while whole-food powders and larger doses, such as some vitamin C or magnesium products, are sometimes given in grams. Always check the unit printed next to the number, not just the number itself.
See every compound we compare and the unit each one uses.
What does NRV mean on a UK supplement label? Is RI the same thing?
NRV stands for Nutrient Reference Value: a fixed daily amount of a vitamin or mineral, set in UK and EU nutrition law, that labels use as a yardstick to show what percentage of it one serving provides.
RI (Reference Intake) is the equivalent term used on ordinary food labels for energy, fat, sugar and salt; supplement labels use NRV specifically for vitamins and minerals, so in practice the two terms cover different sets of nutrients rather than meaning the same thing on the same label. Both exist to give a quick, standardised comparison point rather than a personal recommendation, and the values themselves do not change from product to product.
See our factor table for the amounts behind our own comparisons.
What does 400% NRV mean?
400% NRV means one serving provides four times the fixed Nutrient Reference Value set for that vitamin or mineral, so for a nutrient with an NRV of 25 micrograms, 400% is 100 micrograms.
The percentage is calculated directly from the milligram or microgram amount on the label, using the same fixed NRV table for every brand, so 400% means the same absolute amount whichever product states it. A high percentage on its own says nothing about whether that amount is unusually large in practice: for some vitamins and minerals, hundreds of per cent NRV is a normal, commonly sold dose, because the reference value itself is a small baseline figure rather than an upper limit.
Can a supplement contain more than 100% of the NRV in the UK?
Yes: UK food supplement labelling rules require every label to show the percentage of the Nutrient Reference Value (NRV) each serving provides, but set no maximum, so several hundred per cent NRV on one serving is normal and legal.
This is because the NRV is a reference baseline for labelling, not a safety ceiling, and manufacturers are free to formulate above it. The regulations require the amount to be “expressed also as a percentage … of the relevant recommended daily allowance” (The Food Supplements (England) Regulations 2003, regulation 6(3)(e), checked 22 September 2026). Separate safe upper limits, set by official bodies, are a different figure from the NRV and are not shown as a percentage on the label.
If you take medication, are pregnant, or have a health condition, check with a pharmacist or GP first.
Source: The Food Supplements (England) Regulations 2003, regulation 6(3)(e) (checked 22 September 2026)
What does "elemental" mean for a mineral supplement?
“Elemental” means the actual weight of the mineral itself (for example, magnesium or zinc), separate from the weight of whatever it is chemically bound to in the capsule, such as citrate, glycinate or oxide.
Minerals are never sold as the bare metal; they are bound to another substance to make a stable, absorbable compound, and that compound’s total weight is usually larger than the mineral it contains. “Magnesium citrate 500 mg” describes the compound’s weight, while “providing 80 mg elemental magnesium” describes what your body actually receives. Because forms differ in how much elemental mineral they carry per gram, comparing products by compound weight alone can be misleading; we compare by elemental content wherever a label states or lets us estimate it.
What do "providing", "of which" and "equivalent to" mean on a label?
All three phrases introduce a more specific figure inside a larger one: “providing” and “equivalent to” usually convert a compound’s weight into the active amount it delivers, and “of which” breaks a total down into named parts.
“Magnesium bisglycinate 660 mg, providing 80 mg magnesium” states the compound weight and then the elemental amount it provides. “Omega-3 1,000 mg, of which EPA 300 mg, DHA 200 mg” splits a total oil weight into its measured components. “Ashwagandha extract 500 mg, equivalent to 5,000 mg root” restates an extract in terms of the raw herb it was concentrated from. We read whichever figure is the actual active content, not the largest number printed.
See how we read these figures across the compounds we compare.
What is a serving, and why is it not always one capsule?
A serving is the amount a manufacturer defines as one dose on the label, which can be one capsule, several capsules or tablets taken together, a scoop of powder, or a set number of drops.
Manufacturers choose the serving size that suits their formulation: a nutrient that needs a larger dose is often split across two, three or even six capsules per serving so that each capsule stays a practical size to swallow. This means a “100-capsule” tub is not necessarily 100 servings — if the serving is two capsules, it is 50. We always calculate price per dose from the stated serving, not from the capsule count, so packs with different serving sizes are still compared fairly.
See a pack size example or compare products by dose.
What does "per daily dose" mean?
“Per daily dose” means the amount provided by the total number of capsules, tablets or scoops the label recommends taking across a whole day, which is sometimes more than one serving.
Some products define a single serving as the full daily dose, while others state a serving as, say, one capsule, then separately recommend taking two or three servings a day to reach the intended amount — so “per daily dose” and “per serving” can differ on the same label. We calculate price per dose from whichever figure represents what you actually take in a day, reading the directions alongside the nutrition panel rather than the panel alone, so packs with different serving conventions are still compared like for like.
Compare products by price per dose.
What is a 10:1 extract, and what does the ratio tell me?
A 10:1 extract means it took roughly 10 kg of raw herb to concentrate down to 1 kg of finished extract, so the ratio describes concentration, not the extract’s exact strength.
Extract ratios are useful for comparing concentration methods but do not by themselves state how much of any specific compound, such as withanolides in ashwagandha, ends up in the extract, because plants vary and manufacturers can standardise separately. A “500 mg, 10:1 extract” is sometimes also labelled “equivalent to 5,000 mg root” — the same fact stated two ways. Where a percentage standardisation is also given, that figure is the more precise one to compare on.
See how we compare ashwagandha extracts.
What does "standardised to 5% withanolides" (or any percentage) mean?
A standardised extract has been tested and adjusted so that a named marker compound, such as withanolides in ashwagandha or curcuminoids in turmeric, makes up a guaranteed minimum percentage of the extract’s weight.
Standardisation exists because plant extracts vary naturally between harvests, and a fixed percentage lets a buyer compare batches, and different brands’ products, on a more consistent basis than an unstandardised extract allows. “500 mg extract, standardised to 5% withanolides” means about 25 mg of that 500 mg is the named marker compound. The marker compound is usually one of several active substances in the plant, chosen because it is easy to measure, so the percentage describes that one marker, not the whole extract’s overall composition.
What does "whole herb equivalent" mean?
“Whole herb equivalent” restates a concentrated extract’s strength as the amount of raw, unprocessed herb needed to match it. For example, “500 mg extract, whole herb equivalent to 5,000 mg” means the extract was concentrated from about ten times its own weight of raw herb.
This figure is another way of expressing the same extract ratio (here, 10:1), aimed at readers more familiar with traditional dosing in grams of dried herb than with extract percentages. It describes concentration, not the amount of any single active compound, so two extracts with the same whole herb equivalent can still differ in their standardised marker percentage. We compare products by the extract’s own weight, since that is what is actually in the capsule.
Why does a 1,000 mg fish oil capsule contain less than 1,000 mg of omega-3?
Because “1,000 mg fish oil” states the oil’s total weight, and omega-3 fatty acids (mainly EPA and DHA) are only part of that weight — the rest is other fats naturally present in the source.
A typical standard-strength fish oil capsule might contain 1,000 mg of oil but only around 300 mg of combined EPA and DHA, while a “concentrated” or “triple-strength” oil of the same total weight is refined to carry a much higher proportion. Reading the panel for the EPA and DHA figures specifically, rather than the total oil weight on the front of the pack, is the only way to know the actual omega-3 content, and it is what we rank products on.
What is the difference between EPA, DHA and "total omega-3" on a label?
EPA and DHA are the two specific omega-3 fatty acids most fish and algae oils are valued for, usually listed with their own milligram figures. “Total omega-3” is a wider category that can also include smaller amounts of other, less-studied omega-3 fatty acids.
A label that states EPA and DHA separately lets you add the two figures for the amount we compare products on. A label that gives only a total omega-3 figure, with no EPA or DHA breakdown, tells you less about the oil’s actual composition, since the total could be mostly EPA and DHA or could include a meaningful share of other fatty acids. Where only a total is stated, we use it but mark the product accordingly.
How much elemental magnesium is in 500 mg of magnesium citrate?
About 80 mg: magnesium citrate is roughly 16% elemental magnesium by weight, so 500 mg of the compound provides around 80 mg of magnesium itself.
This is a worked example for one form; the same sum works differently for every other form, because each magnesium compound carries a different share of elemental magnesium; bisglycinate is about 11.7%, oxide about 60.3%. We calculate this conversion automatically for every product from the form stated on its label, using the factors on our methodology page, rather than a single fixed figure for “magnesium” in general. This page is a hand-written example rather than one that regenerates itself daily; the figures behind our price tables always reflect the current factor table.
See magnesium citrate prices per 100 mg elemental magnesium, or every form’s factor.
Why do two magnesium products with the same milligrams on the front differ in elemental magnesium?
Because the large number on the front of a magnesium pack is usually the weight of the whole compound (magnesium bound to citrate, glycinate, oxide and so on), and each compound carries a different proportion of elemental magnesium by weight.
“Magnesium oxide 500 mg” and “magnesium bisglycinate 500 mg” both state 500 mg of compound, but oxide is about 60% elemental magnesium (around 300 mg) while bisglycinate is about 11.7% (around 59 mg) — nearly a fivefold difference from the same headline number. This is why comparing magnesium products by the front-of-pack figure alone can be misleading, and why we rank magnesium products within each form separately, by elemental content where it is stated or can be estimated.
Compare magnesium forms by elemental content.
What does "as ascorbic acid" mean for vitamin C?
“As ascorbic acid” states the vitamin C amount in terms of plain ascorbic acid, the most common chemical form of vitamin C, even when the product actually uses a buffered salt such as sodium ascorbate or calcium ascorbate.
Buffered vitamin C salts weigh slightly more than plain ascorbic acid for the same vitamin C content, because they carry an extra mineral attached; stating the figure “as ascorbic acid” lets every vitamin C product be compared on one consistent basis regardless of which salt is used. So “500 mg vitamin C (as ascorbic acid)” on a calcium ascorbate product means the vitamin C activity is equivalent to 500 mg of plain ascorbic acid, not that the tablet weighs 500 mg overall.
Compare vitamin C products per 1,000 mg.
What is the difference between milligrams of creatine monohydrate and milligrams of creatine?
Creatine monohydrate is creatine bound to one water molecule, which adds a little extra weight. So a label stating the monohydrate’s own weight — the industry-standard way of dosing it — gives a slightly larger number than the creatine content alone.
A typical example is “5 g creatine monohydrate, providing 4.4 g creatine” — the 4.4 g is the actual creatine after removing the bound water’s weight. Because monohydrate dosing by convention already accounts for this and is what published research and product labelling both use, we compare creatine monohydrate products by the monohydrate weight itself rather than converting to pure creatine, matching how the dose is actually measured out.
Compare creatine monohydrate prices per 5 g.
What does "anhydrous" mean?
“Anhydrous” means a compound with no water molecules bound into its structure, as opposed to a “hydrate” form, which has one or more water molecules attached and therefore weighs more for the same amount of active ingredient.
The distinction matters on supplement labels because some minerals and compounds are sold in both anhydrous and hydrated versions, and the water’s weight changes how much of the actual active substance a given number of milligrams contains — an anhydrous form packs slightly more active ingredient per milligram than its hydrate. Creatine is the most common example: “creatine monohydrate” explicitly includes one water molecule, while creatine sold as “anhydrous” has had it removed. Always check which version a label states before comparing weights directly.
How do IU convert to milligrams for vitamin E, and why does the form change the answer?
For natural d-alpha-tocopherol, the most common vitamin E form in supplements, 1 mg equals about 1.49 IU, so 1 IU is about 0.67 mg. Synthetic DL-alpha-tocopherol has a different potency, so the same IU figure converts to a different milligram amount.
This happens because IU measures biological activity rather than weight, and the natural and synthetic forms of vitamin E are not equally potent gram for gram — natural d-alpha-tocopherol is the more potent of the two by this measure. A label stating only “IU” without naming the form leaves the milligram amount uncertain; where a label states both the IU figure and the specific form, we use the conversion that matches it.
Compare vitamin E products, or see our conversion factors.
How do IU convert to micrograms for vitamin A? What does "µg RE" mean?
For preformed vitamin A (retinol), 1 IU equals about 0.3 micrograms, so 1,000 IU is about 300 micrograms. “µg RE” means micrograms of Retinol Equivalent, the standard unit UK and EU labels use to state vitamin A regardless of its source.
Retinol Equivalents exist because vitamin A on a label can come from preformed retinol or from plant carotenoids such as beta-carotene, which the body converts to retinol only partially and at a variable rate; RE puts both sources on one comparable scale rather than converting each separately. Because the retinol-to-carotenoid conversion is not a single fixed number, a beta-carotene product’s IU-to-microgram conversion differs from a retinol product’s, so always check which source a label states.
What does "DFE" mean for folate?
DFE stands for Dietary Folate Equivalent, the standard unit used to state folate amounts on a comparable scale regardless of whether it comes from food folate or the synthetic folic acid used in supplements.
DFE exists because synthetic folic acid is absorbed more efficiently than the folate naturally found in food, so a milligram-for-milligram comparison would understate folic acid’s actual effect; by convention, 1 microgram of folic acid taken with food counts as 1.7 micrograms DFE. Supplement labels sold in the UK do not always print the DFE figure, sometimes stating only the folic acid or methylfolate weight directly, so check which figure a specific label gives before comparing two products.