When a supplement label says "500 mg magnesium glycinate," it means 500 mg of the compound — not 500 mg of magnesium. Magnesium glycinate (bisglycinate) is roughly 10–14% elemental magnesium by weight; the rest is the glycine backbone that makes it gentle on the gut. That chemistry matters: a "500 mg" capsule may deliver under 70 mg of actual magnesium.