This breaks a compound down into each element’s percentage share of the total mass — the same information a lab’s elemental analysis report gives.
How it works
Mass percent
element’s mass % = (element’s total mass in the formula ÷ molar mass of the compound) × 100
Uses the identical formula parser and IUPAC atomic weights the molar mass calculator already established, so the two can never disagree on what a formula parses to.
Why the percentages always sum to 100%
Every atom in the formula is accounted for in exactly one element’s share — nothing is left out and nothing is double-counted, so the percentages sum to exactly 100% by construction. That’s a useful built-in check: if a computed set of percentages doesn’t sum close to 100, something upstream — most likely the formula itself — is wrong.
How to use this calculator
- Type the compound’s formula, using correct capitalisation.
- Read each element’s percentage of the total mass, heaviest contributor first.
Frequently asked questions
Why is oxygen such a large percentage of water’s mass despite there being only one oxygen atom?
Because oxygen’s atomic weight (16.00) is so much larger than hydrogen’s (1.01) that one oxygen atom outweighs two hydrogen atoms combined — mass percent depends on atomic weight as much as atom count, not on count alone.
How is this used in a real lab setting?
Elemental analysis measures the actual mass percentages present in a sample; comparing that measurement against the percentages this calculator predicts for a proposed formula is one way chemists check whether an unknown compound matches a hypothesised structure.
Does this tell me the empirical formula from percentages?
No — this goes the other direction, from a known formula to percentages. Working backward from measured percentages to an empirical formula is a related but separate calculation this page doesn’t perform.
Why might the percentages not sum to exactly 100.00%?
Rounding in the displayed decimal places, not an error in the underlying calculation — the true, unrounded percentages always sum to exactly 100%.
Can I use this for a hydrate?
Only if you expand the hydrate’s water into its own atom counts within the formula you enter, since the dot notation for hydrates (like ·5H2O) isn’t parsed directly.
