Labels whose declared salt will not fit the capsule

• 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 0.6–0.8 g/mL.
• Where a salt is hydration-ambiguous, the audit takes the reading most favourable to the product — the smallest compound mass the label allows. A label listed below therefore exceeds capacity under every reading, not just the unfavourable one.
• 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 297 of 716 labels.

What this does and does not say: it says the declared salt cannot be the only source of magnesium at the declared fill. It does not say the bottle is mislabelled, unsafe, or that anything was measured in a laboratory. The ordinary explanation is buffering with a denser salt — most often oxide — or a non-standard capsule.

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.