What is percent composition by mass?

Percent composition by mass is the percentage of the total molar mass of a compound contributed by each element. It is calculated as (mass of element in 1 mol of compound / molar mass of compound) × 100%.

Why do the percentages not always add up to exactly 100%?

Rounding each percentage to two decimal places can introduce small discrepancies. The underlying calculation always sums to 100%; any apparent gap is a display rounding artifact.

How do I write formulas with parentheses or hydrates?

Use standard chemical notation: Ca(OH)2, Al2(SO4)3, CuSO4·5H2O (enter the dot as a period, e.g. CuSO4.5H2O). Square brackets [ ] and curly braces { } are also accepted.

Which atomic masses does this tool use?

Atomic masses are from the IUPAC 2021 standard atomic weights, covering all 118 elements from hydrogen (1.008 g/mol) to oganesson (294 g/mol).

Can I use this to find the empirical formula?

This tool calculates percent composition from a known formula. To go in the reverse direction (from percentages to formula), you would use an empirical formula calculator instead.

What does the bar chart next to each percentage represent?

The horizontal bar is scaled so that 100% fills the full width. It gives a quick visual sense of which element dominates the compound's mass.

Percent Composition Calculator

Name: Percent Composition Calculator
Availability: InStock
Author: Nham Vu

Enter a chemical formula to instantly see the percent composition by mass of every element in the compound.

Chemical Formula

Formula

Common Compounds

Formula Tips

Capitalize element symbols: NaCl, not nacl
Use parentheses for groups: Ca(OH)2
Hydrates with a period: CuSO4.5H2O
Brackets work too: Al[OH]3

Percent Composition

Live

Enter a chemical formula to see its composition.

Try H2O, Ca(OH)2, or C6H12O6.

Compound

Molar Mass

— g/mol

Mass Percent by Element

Element	Atoms	At. Mass	Subtotal (g/mol)	Mass %
Total				100.00%

Summary

Enter a chemical formula to instantly see the percent composition by mass of every element in the compound.

How it works

Type a chemical formula such as H2O, NaCl, or Ca(OH)2 in the input field.
The parser identifies every element symbol and its subscript count, handling nested parentheses.
Each element's subtotal mass is calculated as: atom count × atomic mass.
Total molar mass is the sum of all element subtotals.
Percent composition for each element = (element subtotal / total molar mass) × 100.
Results are displayed in a ranked table with a visual bar showing relative mass contribution.

Use cases

Solving stoichiometry and percent-composition problems in general chemistry courses.
Verifying the theoretical composition of a synthesized compound.
Determining the mass of a specific element present in a given sample.
Comparing elemental ratios across similar compounds (e.g. different oxides).
Teaching aid to illustrate how atomic mass drives composition percentages.
Pharmaceutical chemistry: checking active-ingredient elemental makeup.
Environmental science: understanding carbon or nitrogen content of molecules.
Cross-checking published compound data against calculated values.

Frequently Asked Questions

Last updated: 2026-06-09 · Reviewed by Nham Vu