The elemental calculator.
"Magnesium oxide 400 mg" is not 400 mg of magnesium - only part of a mineral compound's weight is the mineral itself. Enter a compound and amount to estimate the elemental mineral you actually get.
Free access to the Elemental calculator.
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Method & sources
Elemental amount = compound amount × (elemental % ÷ 100). The percentages come from each compound's molar mass - the share of its total weight that is the mineral - cross-checked against NIH Office of Dietary Supplements fact sheets (e.g., magnesium oxide ~60%, magnesium glycinate ~14%, calcium carbonate 40%, ferrous fumarate ~33%, zinc oxide ~80%). As-sold hydrated salts can run a few points lower - magnesium citrate, often ~11–12% as the nonahydrate actually sold, is the classic example - so when a label states an elemental amount, that number wins.
- NIH Office of Dietary Supplements - Magnesium, Calcium, Iron, Zinc, Potassium fact sheets (elemental % by form).
- IUPAC - Standard Atomic Weights (2021).
- FDA Daily Values - 21 CFR 101.9 (Mg 420 mg, Ca 1300 mg, Fe 18 mg, Zn 11 mg, K 4700 mg).
Compound weight vs. elemental weight.
Minerals are sold as compounds - magnesium bound to an oxide, glycine, or citrate molecule, for example. Only part of the compound's total weight is the mineral itself. That fraction is the "elemental" amount, and it is the number that actually matters.
A clear example: magnesium oxide is about 60% magnesium by weight, so "400 mg magnesium oxide" is roughly 240 mg of elemental magnesium on paper. Magnesium bisglycinate is only about 14% magnesium by weight - but it is far better absorbed, so a smaller elemental number can still deliver more usable magnesium. Weight on the label and absorption in your body are two different things.
What an honest label does
A transparent label states the elemental amount directly - for example, "Magnesium (as bisglycinate) 150 mg." If a label only shows the compound weight, you cannot tell how much mineral you are really getting without an estimate like this one.
What this tool isn't
It is an educational estimate based on standard chemistry, not a precise measurement of a specific product and not dosing advice. For how much of a mineral is right for you, talk to a healthcare provider.