Mass percent (also called percent composition when applied to a compound's elements, or percent by mass / w/w% when applied to a solution) is one of the most common ways chemistry expresses how much of something is present relative to the whole. It shows up in everything from balancing chemical formulas to reading a nutrition label.

How mass percent works

Mass percent is always the same ratio: the mass of the part you care about, divided by the mass of the whole, multiplied by 100. For an element in a compound, the part is how much mass that element contributes per mole of the compound, and the whole is the compound's total molar mass — both come from the periodic table and the compound's formula, so no lab measurement is needed. For a solute in a solution, the part and whole are both measured masses: how much solute you dissolved, and how much the whole solution weighs once mixed.

Applying this formula to every element in a compound gives its full percent composition — a breakdown that always adds up to 100% across all elements. This is the same calculation used to derive an empirical formula from experimental mass data.

Inputs and what they mean

On the Element in Compound tab, enter the mass (in g/mol) that one element contributes to one mole of the compound, and the compound's total molar mass (also g/mol). For CO₂, carbon contributes 12 g/mol and the compound's molar mass is 44 g/mol (12 + 16 + 16).

On the Solute in Solution tab, enter the solute's mass and the total solution mass, both in grams — the solution mass already includes the solute, so it's solute + solvent, not solvent alone. The Solve tab flips either calculation around: give it a target mass percent and one known mass, and it finds the other.

Limits and edge cases

Mass percent by itself doesn't tell you moles, volume, or molarity — converting between mass percent and molarity requires the solution's density, which this calculator doesn't ask for. A whole mass of zero is undefined and is guarded against; a negative mass isn't physically meaningful and is rejected.

For a compound's full percent composition, run this calculator once per element and confirm the percentages sum to 100% (small rounding differences are normal). If you're going the other direction — deriving a compound's formula from percent composition data — use an empirical formula calculator instead, since that requires converting each percent to moles.