Estimated salt mass versus assumed capsule capacity

• A US Supplement Facts panel declares minerals as the element. The salt carrying that element is heavier — magnesium oxide is 60.3% magnesium by mass, magnesium glycinate 14.1%, magnesium gluconate 5.9%.
• Dividing the declared elemental milligrams by the salt's fraction gives the mass of salt the serving must contain. Dividing again by the units per serving gives the mass per capsule.
• Standard capsules hold roughly 545 mg (size 0), 730 mg (size 00) and 1,100 mg (size 000) at powder density 0.8 g/mL, the top of the usual 0.6–0.8 range.
• Where a salt is hydration-ambiguous, the audit takes the reading most favourable to the product — the smallest compound mass the label allows, divided across the largest number of units its serving allows (a “1–2 capsules” serving is read as two). A label listed below is above this assumed capacity even at that low-mass end; a different density or capsule volume can change the comparison.
• Labels naming more than one salt, or naming a class such as “amino acid chelate” rather than a compound, are excluded: no single fraction applies. That is 359 of 716 labels.

The comparison assumes the entire capsule volume is available to the magnesium compound. Fillers occupy additional volume; actual density, tamping and capsule dimensions have not been measured. Exceeding this model does not establish a label error.

Why some capsule labels have no single-salt capacity verdict

As of 2026-10-17, 210 capsule or softgel labels are excluded from the single-salt capacity comparison because the elemental amount, composition or hard capsule model is unresolved.

At powder density 0.8 g/mL and assumed size 000 capacity of 1,100 mg, 8 of 137 audited hard capsule labels are above the model on 2026-10-17.

Percent daily value is recomputed on each product page against the 420 mg reference in 21 CFR 101.9. Source records: NIH Dietary Supplement Label Database, snapshot 2026-08-10.

165 capsule labels name a blend, a class such as amino acid chelate, several amounts or no form at all, so no single-salt verdict exists for them; at the most favourable salt they could hold, 0 exceed size 000, against 8 of 137 single-salt labels. Lower bound, not a measurement: the salt richest in magnesium among those named, or magnesium oxide when the complete set of carriers is unresolved.

Size 0 at the bound: 130 · size 00: 17 · size 000: 18. A bound that fits does not show that the real salt mix fits.

3 statements imply a magnesium share above the corresponding pure-salt reference on 2026-10-17; a named complex has unresolved composition, so its ratio does not establish the composition of that complex.