Magnesium carbonate

A US Supplement Facts panel declares the element, not the salt. Magnesium carbonate has a molar mass of 84.31 against magnesium's 24.305, so 24.78–28.83% of its mass is magnesium. One caveat: anhydrous or hydrated; labels rarely state which — so the figures below are given as a range wherever that matters.

A size 00 capsule holds about 730 mg of powder at powder density 0.6–0.8 g/mL; a size 000 about 1,100 mg.

Absorption is a separate question from mass, and a contested one. Below is what each study did, on whom, and what it measured — with the original linked. See the full literature page for why results conflict.

These entries report what each study measured and on whom. They are not advice, not a ranking, and not a claim about any product on this site. Every reference links to its PubMed record so the original can be read in full.

1. Effects of short-term magnesium supplementation on ionized, total magnesium and other relevant electrolytes levels.

   Biometals : an international journal on the role of metal ions in biology, biochemistry, and medicine · 2022 · PMID 35041150

   Design

   Short-term, 10-day supplementation study in 61 healthy young female adults, comparing three magnesium supplement forms: magnesium oxide (powder/granulate), magnesium citrate, and magnesium carbonate (effervescent tablets).

   What it measured

   Ionized magnesium concentration and its percentage of total magnesium; serum total magnesium concentration; other serum electrolyte levels — each assessed at baseline and after the 10-day intervention.

   Limits

   Healthy young female adults only, no male or older/deficient populations studied; short 10-day intervention with no longer-term follow-up; three specific product formulations were tested, so results may not generalize to other products of the same salts; the abstract does not report the per-arm breakdown of the 61 participants.

2. Study of magnesium bioavailability from ten organic and inorganic Mg salts in Mg-depleted rats using a stable isotope approach

   Magnesium Research · 2005 · PMID 16548135

   Design

   Eighty male Wistar rats fed a magnesium-depleted diet for three weeks, then randomised into ten groups, each receiving a different magnesium salt labelled with a stable isotope.

   What it measured

   Stable isotopes allow absorption and retention to be traced directly rather than inferred from urine, across ten salts at once — the widest head-to-head comparison in the literature.

   Limits

   Rats, not people, and magnesium-depleted rats at that: depletion itself raises absorption. The breadth of the comparison is its value; the species is its limit.

Compare carbonate with other salts

13 published comparisons include magnesium carbonate as of 2026-09-13; each row gives the other salt's magnesium fraction and compound mass for 100 mg elemental magnesium from the chemistry model.

• Magnesium carbonate vs magnesium citrate: 11.3–16.16% magnesium by mass; 619–885 mg compound per 100 mg elemental magnesium.
• Magnesium carbonate vs magnesium glycinate: 11.83–14.1% magnesium by mass; 709–845 mg compound per 100 mg elemental magnesium.
• Magnesium carbonate vs magnesium oxide: 60.32% magnesium by mass; 166 mg compound per 100 mg elemental magnesium.
• Magnesium carbonate vs magnesium malate: 13.2–15.54% magnesium by mass; 644–758 mg compound per 100 mg elemental magnesium.
• Magnesium carbonate vs magnesium aspartate: 8.42% magnesium by mass; 1,188 mg compound per 100 mg elemental magnesium.
• Magnesium carbonate vs magnesium gluconate: 5.86% magnesium by mass; 1,706 mg compound per 100 mg elemental magnesium.
• Magnesium carbonate vs magnesium chloride: 11.96–25.53% magnesium by mass; 392–836 mg compound per 100 mg elemental magnesium.
• Magnesium carbonate vs magnesium taurate: 8.85% magnesium by mass; 1,130 mg compound per 100 mg elemental magnesium.
• Magnesium carbonate vs magnesium hydroxide: 41.68% magnesium by mass; 240 mg compound per 100 mg elemental magnesium.
• Magnesium carbonate vs magnesium l-threonate: 8.25% magnesium by mass; 1,212 mg compound per 100 mg elemental magnesium.
• Magnesium carbonate vs magnesium orotate: 7.27% magnesium by mass; 1,376 mg compound per 100 mg elemental magnesium.
• Magnesium carbonate vs magnesium lactate: 12.01% magnesium by mass; 833 mg compound per 100 mg elemental magnesium.
• Magnesium carbonate vs magnesium sulfate: 9.86–20.19% magnesium by mass; 495–1,014 mg compound per 100 mg elemental magnesium.

Excipients most common in carbonate labels

5 published excipient facets below have the highest frequencies among 23 labels mentioning magnesium carbonate in the NIH DSLD records, counted 2026-09-13; each label enters an ingredient count once.

• silicon dioxide: 11 of 23 labels.
• cellulose: 10 of 23 labels.
• citric acid: 7 of 23 labels.
• magnesium stearate: 6 of 23 labels.
• malic acid: 6 of 23 labels.