Most of what a 325mg iron tablet weighs is not iron
The number printed largest on the bottle is the weight of the whole compound. A standard 325 milligram ferrous sulfate tablet delivers 65 milligrams of elemental iron.
What the study found
A 325mg iron tablet contains 65mg of iron
The number printed largest on a supplement label is the weight of the whole compound, not the weight of the iron in it. A standard 325 milligram ferrous sulfate tablet delivers 65 milligrams of elemental iron, which the same label states as 361% of the Daily Value.
The caveat · One label; other iron salts differ
This is one manufacturer's label. Other iron salts — fumarate, gluconate, bisglycinate — deliver different amounts of elemental iron per tablet, so the ratio here is not a conversion factor for every product. The label itself carries three disagreeing revision dates, so treat it as a record of what a shopper is told rather than of when they were told it.