Percent Composition Calculator

01 Set up your calculation

1. Enter a single chemical formula using case-sensitive element symbols.
2. Use parentheses for repeated groups and a dot for water of crystallization.
3. Review the total and each element’s mass share; the percentages sum to 100.

02 The relationship

For the assumptions behind these results, read the explanation below. You can also explore Degree of Unsaturation and Molar Mass.

03 Mass percentage is not atom percentage

The element count tells you how many atoms occur, but heavier atoms contribute more mass. Water has twice as many hydrogen atoms as oxygen atoms, yet oxygen contributes most of its mass.

This calculation uses a formula as written. It does not identify a compound by name, balance an equation, or infer a missing subscript. Check the displayed atom count if a result looks unexpectedly large.

04 Write a formula the parser can read

Use proper element capitalization: Co is cobalt, while CO contains carbon and oxygen. Ordinary digits and subscript digits work. Parentheses and square brackets can be nested, as in Al2(SO4)3 or K4[Fe(CN)6].

Separate hydrate components with a middle dot or period, for example CuSO4·5H2O. The number after the dot multiplies that entire component. Do not put a reaction coefficient before the first formula.

05 Understand the reported precision

The element values are rounded conventional atomic weights for ordinary material. Natural isotope composition varies, so the final digits should not be interpreted as a sample-specific measurement.

Charge notation, explicit isotope labels, fractional occupancies and full reaction equations are unsupported. Elements without a conventional standard atomic weight require a separately supplied isotope-specific mass in a suitable mass-conversion tool.