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Elemental Mineral Calculator – Magnesium, Calcium, Zinc and Iron

Convert a mineral compound on a supplement label into the amount of elemental magnesium, calcium, zinc or iron with this elemental mineral calculator.

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For information only, not medical advice. Reference values describe healthy population groups and do not tell you what dose you should take. Ask a doctor, pharmacist or dietitian before starting, stopping or combining supplements, especially if you are pregnant, take medication or have a health condition.

Reference values are for adults aged 19 and over. Children have different, lower limits. Sources: US Dietary Reference Intakes (NIH Office of Dietary Supplements) and EFSA Dietary Reference Values and Tolerable Upper Intake Levels (2024). Reviewed October 7, 2026.

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How to use the Elemental Mineral Calculator

  1. 1 Choose the compound named on the label, for example calcium carbonate or ferrous sulfate.
  2. 2 Enter the weight of the compound per tablet or serving in milligrams, and the number of servings per day.
  3. 3 Read the elemental amount and compare the daily total with the US and EU upper limits.

Compound weight versus elemental amount

Minerals in supplements are bound to another molecule, such as carbonate, citrate or gluconate. The total weight of that compound is much higher than the amount of the mineral itself. Reference intakes and upper limits always refer to the elemental mineral, so a label or prescription that gives the compound weight has to be converted.

The elemental share follows from the chemical formula: the atomic mass of the mineral divided by the molar mass of the compound. The calculator uses the formula shown in the result, including water of crystallisation where a compound is normally sold as a hydrate.

Typical elemental shares

Compound Elemental share
Magnesium oxide 60%
Magnesium citrate 16%
Magnesium glycinate 14%
Calcium carbonate 40%
Calcium citrate (tetrahydrate) 21%
Zinc gluconate 14%
Zinc citrate 31–32%
Ferrous sulfate (heptahydrate) 20%
Ferrous fumarate 33%
Ferrous gluconate 12%

So 1,250 mg of calcium carbonate provides 500 mg of calcium, and a 325 mg ferrous sulfate tablet provides about 65 mg of iron.

Why the form matters

The elemental share is not the same as how well a mineral is absorbed. Calcium citrate contains less calcium per tablet than carbonate but is absorbed well without food; magnesium oxide is rich in magnesium but less soluble. Manufacturers sometimes use slightly different hydrates or chelates, so the percentages on a specific product can differ by a few percent. When a label already states “calcium (as carbonate) 500 mg”, the 500 mg is the elemental amount and no conversion is needed.

Upper limits for elemental minerals

The result compares the daily elemental amount with the adult upper limits: for magnesium 350 mg (US) or 250 mg (EU) from supplements, calcium 2,500 mg, zinc 40 mg (US) or 25 mg (EU), and iron 45 mg (US) or a safe level of 40 mg (EU). Prescribed iron for anaemia often exceeds these values under medical supervision.

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Frequently asked questions

How much elemental iron is in 325 mg ferrous sulfate?

About 65 mg. Ferrous sulfate heptahydrate is roughly 20% iron.

How much calcium is in 1,250 mg of calcium carbonate?

About 500 mg. Calcium carbonate is 40% calcium.

Why does my product give a slightly different percentage?

Chelates and hydrates vary between manufacturers. The calculator uses the standard formula shown in the result; the label’s own elemental amount takes precedence.

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