Magnesium is sold in forms, and the form decides how much magnesium you buy. The number on the front usually refers to the compound, not to the mineral.
Magnesium bisglycinate is 14.1 percent elemental magnesium: molar mass 172.4 g/mol, of which 24.3 grams is magnesium. A 500 milligram capsule of bisglycinate therefore delivers 70.5 milligrams of magnesium. Oxide is 60.3 percent, citrate 16.2 percent.
Below is what a daily dose costs, with the elemental content beside it. Nothing here about what magnesium does: outside the fixed EU list of authorised claims everything is prohibited on a page carrying product links, and the sum does not need it.
