Convert the stated mass of a specific mineral compound into its approximate elemental magnesium, iron or zinc content, then compare it with the UK labelling NRV.
This free calculator uses molecular formulae and molecular weights for eight clearly named compounds. It is designed to explain labels and technical specifications, not to tell you how much of a supplement to take.
Check the label before calculating
If a label says “Magnesium 100 mg (as magnesium citrate)”, the 100 mg is already the declared amount of magnesium. Do not convert it again.
Use this tool only when you have been given the mass of the compound itself — for example, a specification that states 500 mg of magnesium oxide — and you know the exact chemical form.
Why “elemental” amount matters
A mineral ingredient contains the named mineral plus the other parts of its chemical compound. The compound weight and the elemental mineral weight are therefore different. The calculator divides the mass contributed by magnesium, iron or zinc atoms by the molecular weight of the complete compound, then applies that proportion to the amount entered.
For example, pure magnesium oxide has the formula MgO. Using the relative masses in that formula, magnesium accounts for about 60.30% of the compound by mass. A stated 500 mg of pure magnesium oxide therefore contains a theoretical 301.5 mg of magnesium.
Why the exact form matters
Water molecules can be part of a compound's crystal structure. That is why an anhydrous form and a hydrated form can contain different proportions of elemental mineral even when their everyday names sound similar. “Magnesium citrate” alone is not specific enough for a universal conversion, so this calculator includes only the clearly defined tribasic nonahydrate form.
Commercial ingredients may also be buffered, blended, diluted or supplied to an assay specification that differs from the theoretical pure compound. If the product specification, certificate of analysis or manufacturer gives an elemental assay, use that verified figure rather than this theoretical calculation.
About the NRV result
The UK labelling Nutrient Reference Values used here are 375 mg for magnesium, 14 mg for iron and 10 mg for zinc. An NRV percentage is a standard labelling comparison for adults; it is not a personal dose target, a maximum intake or a measure of product quality.
Formula and molecular-weight sources
The molecular formulae and molecular weights are taken from the US National Library of Medicine's PubChem database. Theoretical percentages are calculated from the formulae shown there and standard atomic weights.
- Magnesium: magnesium oxide, magnesium glycinate, magnesium glycinate dihydrate and magnesium citrate nonahydrate.
- Iron: ferrous fumarate.
- Zinc: zinc oxide, zinc gluconate and zinc picolinate.
- UK NRVs: Annex XIII of assimilated Regulation (EU) No 1169/2011 and the GOV.UK food-supplement labelling guidance.
Reviewed: 13 September 2026.
Related free resources
Use the UK Supplement Label Checklist →
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Use the Cost and Reorder Calculator →
Important: This is a chemistry and label-reading tool, not medical advice. It does not recommend a dose, assess absorption or decide whether a supplement is suitable for you. Follow the product label and ask a pharmacist, GP or other qualified healthcare professional when appropriate.