Percent Composition Calculator

Percent composition answers a different question from atom counting: how much of the compound’s mass comes from each element? The calculator first determines the molar-mass contribution of every element, then divides each contribution by the total molar mass.

Two elements can have the same atom count and very different mass percentages because their atomic weights differ. The calculation uses the same reference atomic weights as the existing Mendeleev molar-mass tool and stops rather than inventing a value when no standard atomic weight is available.