Supplement Labels Explained UK: What NRV, mg, mcg, IU and "Per Serving" Really Mean

What is a supplement label actually telling you?

A UK food supplement label is essentially a small nutrition panel. In one place it tells you which nutrients are inside, how much of each you get, and how that compares to a standard reference intake. Once you know how to read those three things, most of the confusion disappears.

Every label has the same building blocks:

• The ingredient or nutrient name — sometimes with its specific form in brackets, such as vitamin B12 (methylcobalamin) or magnesium (as magnesium glycinate). The form can affect how the nutrient behaves, which is why a quality label states it.
• The amount per serving — a number plus a unit (mg, mcg/µg or IU).
• The %NRV — how that amount compares to the Nutrient Reference Value for an average adult.
• The serving size — how many tablets or capsules count as "one serving".

A broad multivitamin is the easiest place to see all of this at once. If you look at an A–Z multivitamin label, you'll see a long list of nutrients, each with its amount and its %NRV next to it — a useful "Rosetta Stone" for everything below.

mg vs mcg vs IU: what do the units mean?

The quickest way to misread a label is to compare two different units as if they were the same. Here's the plain-English version.

Milligrams (mg) and micrograms (mcg / µg)

These are units of weight. A milligram is one-thousandth of a gram, and a microgram is one-thousandth of a milligram. So:

• 1 mg = 1000 mcg
• "mcg" and "µg" are exactly the same thing — both mean micrograms. Some labels use the "µg" symbol, others spell out "mcg" because µ can be hard to read or print.

This matters because nutrients are dosed at very different scales. Vitamin C is usually measured in milligrams, while vitamin B12 and biotin are measured in micrograms. A label that says 1000µg of vitamin B12 is providing 1 mg — it only looks like a huge number because it's written in the smaller unit.

International Units (IU)

IU is not a unit of weight — it's a measure of biological activity, used mainly for fat-soluble vitamins like vitamin D, vitamin A and vitamin E. Because IU is activity-based, the conversion to micrograms is different for each vitamin. For vitamin D the standard conversion is:

• 1 mcg of vitamin D = 40 IU, so 25 mcg = 1000 IU and 100 mcg = 4000 IU.

That's why a product such as a vitamin D3 supplement labelled 4000 IU per serving is the same as 100 mcg. If you ever need to compare a product listed in IU with one listed in mcg, convert them to the same unit first.

What does %NRV mean on UK supplements?

%NRV stands for "percentage of the Nutrient Reference Value". The NRV is a standard daily reference amount for an average adult, set so that labels can be compared consistently. The percentage simply tells you how much of that reference one serving provides.

So a label showing "Vitamin C 80 mg (100% NRV)" means one serving gives you 100% of the reference intake for vitamin C. A few practical points:

• It's a comparison tool, not a strict target. Some nutrients are deliberately included at well above 100% NRV (this is allowed within safe limits), while others are below it because you also get them from food.
• Some nutrients have no NRV. Ingredients like botanical extracts, probiotics or amino acids often show "no NRV established" or a dash — that's normal, not a quality flaw.
• NRV is a general reference, not a personalised dose. Your needs can differ with age, diet, life stage and health, which is where label-led routines and professional advice come in.

For context on why reference intakes exist and how everyday diet fits in, the NHS guide to vitamins and minerals is a reliable starting point.

Why "per serving" is the number that matters

This is the single most common source of confusion. The amounts on a label are usually given per serving — and one serving is not always one tablet or capsule.

Imagine two magnesium products:

• Product A: "Magnesium 1000 mg per serving — serving size 2 capsules."
• Product B: "Magnesium 500 mg per serving — serving size 1 capsule."

Per capsule these are identical (500 mg each). The "1000 mg" on Product A simply reflects a two-capsule serving. Our own magnesium glycinate label states 1000 mg per serving precisely so you can see the full serving amount — but you should always check the serving size to compare like with like.

"Per serving" also affects how long a pack lasts and its true value. A 90-capsule tub at two capsules per serving is a 45-day supply, not 90 days. Reading the serving size is how you compare both strength and cost fairly.

How to compare two supplements step by step

Here's a simple routine you can use on any product page or pack:

1. Match the units. Convert everything to the same unit (mg, mcg or IU) before you compare. Remember 1 mg = 1000 mcg, and for vitamin D 1 mcg = 40 IU.
2. Find the serving size. Note how many tablets or capsules make one serving, then work out the amount per single tablet or capsule.
3. Check the %NRV (where one exists) to see how the amount compares to the reference intake.
4. Look at the nutrient form. The bracketed form (e.g. methylcobalamin, magnesium glycinate) tells you what you're actually taking.
5. Work out the real value. Divide the price by the number of servings, not the number of tablets.

Run two products through these five steps and you'll usually see straight away which one genuinely offers more, and which just looks bigger because of how it's written. If you'd rather start from a clearly labelled range, you can browse the full Vita London range and read each label the same way.

Does "high strength" mean better?

Not necessarily. "High strength" is a marketing description, not a regulated guarantee that a product is right for you. For some nutrients, more is genuinely useful up to a point; for others, going far above the reference intake adds cost without adding benefit, and a few have safe upper limits worth respecting.

A balanced way to think about it:

• Water-soluble nutrients (like vitamin C and the B vitamins) are generally well tolerated, and any excess is mostly passed in urine — but that also means very high doses can simply be wasted.
• Fat-soluble vitamins (A, D, E, K) and some minerals can build up, so it's sensible to stay within label guidance and not stack several products that contain the same nutrient.
• "High strength" only helps if you actually need that amount. A combined product such as a vitamin C & zinc supplement may give you a sensible everyday amount rather than the highest possible number — which can be exactly what you want.

The NHS pages on individual nutrients, such as vitamin D and the B vitamins, are a good place to sense-check whether a "high strength" claim is meaningful for a given vitamin.

Common label-reading mistakes (and how to fix them)

• Comparing mcg with mg. Fix: convert to the same unit first (1 mg = 1000 mcg).
• Treating IU like a weight. Fix: convert IU to mcg using the vitamin-specific factor (1 mcg vitamin D = 40 IU).
• Ignoring the serving size. Fix: always check how many tablets or capsules equal one serving before judging strength or value.
• Assuming a bigger number is stronger. Fix: a large figure may just be the smaller unit (e.g. 1000 µg = 1 mg) or a multi-capsule serving.
• Double-dosing the same nutrient. Fix: if you take more than one product, add up the totals so you don't unintentionally stack, say, zinc from three sources.
• Chasing the cheapest tub. Fix: price per serving, not price per tablet, is the fair comparison.

Frequently asked questions

Is mcg the same as µg?

Yes. Both mean micrograms. "µg" is the scientific symbol and "mcg" is the spelled-out version used on many labels because µ can be misread. They are identical.

How many mcg are in 1 mg?

There are 1000 micrograms (mcg) in 1 milligram (mg). So 1000 µg of a nutrient is the same as 1 mg.

How do I convert IU to mcg for vitamin D?

For vitamin D, 1 mcg equals 40 IU. So 1000 IU = 25 mcg and 4000 IU = 100 mcg. The conversion factor is different for other vitamins, so don't reuse it for vitamin A or E.

What does %NRV mean?

It's the percentage of the Nutrient Reference Value — a standard daily reference for an average adult — that one serving provides. It's there to help you compare products consistently, not as a strict daily target.

Does a higher %NRV mean a better supplement?

Not on its own. Some nutrients are usefully included above 100% NRV, while others sit lower because you also get them from food. Higher is only better if you actually need that amount, and some nutrients have safe upper limits.

Why do some ingredients show "no NRV"?

Because no Nutrient Reference Value has been set for them. This is common for botanicals, probiotics and amino acids, and it doesn't indicate a problem with the product.

Reading a label well is a small skill that pays off every time you shop. Match the units, check the serving size, glance at the %NRV and the nutrient form, and compare by price per serving — and you'll choose supplements with a lot more confidence.