Here is how to read a supplement label: find the declared serving first, then read each nutrient's amount, unit and reference percentage against that serving. Check the ingredient form, full ingredient list and warnings before comparing it with another product.
The largest number on the front may describe something different from the nutrient amount on the back. This guide separates those figures and explains UK label terminology without turning a manufacturer's serving into a personal recommendation.
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In this guide
How do you read a supplement label in order?
A supplement label describes the product's ingredients and their declared quantities, with a portion to which those quantities relate. Read the portion before the numbers. Then identify the nutrient or substance, its unit and any reference percentage. The ingredient list and warnings provide context that a front-label headline cannot.
UK packs do not have to look like a US Supplement Facts panel. DHSC guidance explains the specific declarations for food supplements, including amounts relating to the manufacturer's recommended daily portion. Read the actual heading: per capsule, per serving and per daily portion are not interchangeable.
Find the serving or daily portion
Identify how many capsules, tablets or other units the figures cover. Keep that stated basis beside the nutrient amounts when making notes.
Check how long the pack represents
Use the pack quantity and the declared daily serving to calculate servings per container where this is clear. Do not assume a capsule count is a day count.
Read the nutrient and its unit together
Match each number with its row label. An amount for the whole blend or compound is not automatically the amount of an individual nutrient.
Finish with ingredients and warnings
Read the entire ingredient list, allergen information and pack warnings. A short front-label description cannot replace those details.
What does NRV mean on a UK supplement?
NRV means Nutrient Reference Value. It is a standard labelling reference for listed vitamins and minerals, used to express a declared amount as a percentage. It is not a personalised requirement or an upper limit. A larger percentage is therefore not a reason, by itself, to favour one product.
Annex XIII of the food-information rules lists magnesium's NRV as 375 mg. In our fictional label, 75 mg divided by 375 mg equals 20%. That calculation explains the percentage; it says nothing about the amount an individual should choose.
Not every ingredient has an NRV. A dash or 'NRV not established' for an ingredient without a listed reference is not the same as zero content. Keep reference percentages separate from the quantities actually disclosed on the label.
Why can the active amount be lower than the headline?
A mineral compound includes the mineral and other chemical components. Magnesium citrate is not entirely magnesium; ferrous sulfate is not entirely iron. When comparing mineral quantities, look for the declared magnesium or iron amount rather than using the mass of the whole source compound.
DHSC labelling guidance distinguishes the amount of a mineral from the compounds providing it. If a label names a source without clearly stating the nutrient amount, do not estimate a conversion from a generic percentage. The exact formulation may be unclear.
See how magnesium labels compare
The live cards below connect label reading with product analysis. Check active form, daily dose and testing alongside the price rather than choosing by one headline number.
WeightWorld Magnesium 4-in-1View 
WeightWorld Magnesium Glycinate + B6View 
Zipvit Magnesium Glycinate + B6View We may earn a commission if you buy through our links. How we stay independent.
How do mg, mcg and IU differ?
| Unit | Meaning | Comparison check |
|---|---|---|
| mg | Milligram: a unit of mass | Keep the named ingredient and serving basis attached. |
| mcg or µg | Microgram: a smaller unit of mass | 1 mg equals 1,000 mcg. |
| IU | International Unit: a measure of biological activity | The conversion depends on the substance; there is no universal mg conversion. |
The NHS vitamin D page explains that a microgram is a thousandth of a milligram and that 1 mcg of vitamin D equals 40 IU. That vitamin D conversion is specific to vitamin D. Do not copy it across to another vitamin.
Our Vitamin D comparison shows IU and micrograms together. When checking a label against a listing, compare both the units and the declared daily serving: matching numbers in different units are not matching amounts.
What do the ingredient list and blends tell you?
Ingredient form
Note the source form named beside the nutrient or in the ingredients. Keep the form and amount as separate comparison fields.
Other ingredients
Read capsule shells, coatings, flavourings and other listed components too. 'Other' does not mean irrelevant to dietary preferences or allergens.
Blends
If only a combined amount is disclosed, you cannot work out each ingredient's amount. A blend name cannot fill that gap.
A proprietary blend is a mixture presented under a supplier's own name or formula. For comparison, the key issue is disclosure: does the label give individual quantities, or only a total? Record what is stated. Do not divide the total equally between the ingredients or assume the most prominent one makes up most of it.
If a shop description and the label photograph disagree, check which pack and formulation each describes. An unresolved mismatch is a reason to ask the manufacturer for clarification, not to select whichever number makes the comparison easier.
How should you read claims and warnings?
Read marketing claims separately from composition. The Great Britain nutrition and health claims register records authorised claims and their conditions for England, Scotland and Wales. Northern Ireland has a different regulatory framework. A permitted claim about a nutrient is not a product-testing certificate or a ranking of brands.
Check the pack's warning statements and storage information as part of reading the label. The Food Standards Agency's consumer guidance also highlights the importance of professional advice where supplements and medicines overlap.
When a label cannot answer your question
If you take medicines or need advice about suitability, speak to a pharmacist or doctor. A comparison table explains product information; it cannot assess your individual circumstances.
How does this site make labels comparable?
The Magnesium comparison separates active form from daily dose, then shows price per day, testing and score. Daily cost depends on the product's recorded serving information, so it can differ from a comparison based only on bottle price.
Our methodology explains the comparison approach, and how we score products explains the assessment behind the overall number. Use the detailed fields to understand differences rather than assuming the largest stated amount must score highest.
Before comparing two labels
- Match the stated serving basis and check servings per pack.
- Compare the same nutrient or active substance, with matching units.
- Separate source-compound weight from declared mineral content.
- Read ingredient forms, allergens, other ingredients and warnings.
- Keep NRV percentages, testing evidence and price as distinct information.
Frequently asked questions
What does NRV mean on a supplement?
NRV means Nutrient Reference Value, a standard reference used on labels for specified vitamins and minerals. The percentage compares the declared nutrient amount with that reference. It is neither a personal target nor an upper limit, and it does not describe the quality of the ingredient form or testing.
What is a serving size?
It is the portion to which a label's declared amounts relate, such as a stated number of capsules. Check whether the panel describes one unit, a serving or the manufacturer's daily portion. Those can differ. Reading the basis correctly allows a fair comparison without treating that portion as personalised advice.
Why does a label say 500 mg but show a lower active amount?
In that hypothetical example, the larger figure might refer to a whole compound or blend rather than the individual nutrient. It might also use a different serving basis. Read the exact row labels and units before deciding. For minerals, look for the declared elemental amount; do not guess a conversion from an unclear headline.
What does IU mean?
IU means International Unit, a measure of biological activity rather than a universal mass unit. Conversion depends on the substance. The NHS states that one microgram of vitamin D equals 40 IU. That relationship helps read vitamin D labels, but it should not be reused as a conversion for other vitamins.
What is a proprietary blend?
It is a mixture presented under a supplier's own name or formula. Check whether the label discloses each ingredient's amount or just the combined total. A total alone cannot establish the individual quantities. For comparison purposes, record that limitation rather than assuming equal proportions or inventing the missing amounts.
Compare the information behind the headline
Choose a category and put ingredient forms, declared daily amounts, testing evidence and daily cost side by side.
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