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Compounds · Chemistry

Molar Mass & Percent Composition Calculator

Parse a chemical formula to calculate molar mass, atom counts, and each element’s mass percentage.

Formula shown Runs locally Reviewed Aug 11, 2026
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Equation guide

Chemical formula and standard atomic weights

Molar mass

M = Σ(nᵢAᵢ)

Element mass percentage

mass %ᵢ = nᵢAᵢ/M × 100
Formula parser

Analyze a chemical compound

How to write a formula
  • Use the element symbol with exact capitalization: NaCl, not NACL or nacl.
  • Type subscripts as ordinary numbers immediately after the element: H2O, CO2.
  • Use parentheses for repeated groups: Ca(OH)2 or (NH4)2SO4.
  • For hydrates, separate the parts with a middle dot or period and include the coefficient: CuSO4·5H2O.

Do not enter state symbols, charges, reaction coefficients, or equations. Atomic weights and percentages are displayed with at most three decimals.

Σ nᵢAᵢ

Molar mass and percent composition will appear here.

Using the model

Symbols, assumptions and limitations

Interpretation notes

Standard atomic weights are used. Parentheses and hydrate notation with a middle dot are supported. Element subscripts, group multipliers, and hydrate multipliers must be positive whole numbers.

Worked examples

See the method in practice

01

Calcium hydroxide

Ca(OH)₂ contains one Ca, two O, and two H atoms per formula unit.

02

Copper sulfate pentahydrate

CuSO₄·5H₂O includes five waters of hydration in its molar mass.

Questions

Frequently asked

Can I use parentheses?

Yes. Formulas such as Ca(OH)2 are expanded before the mass is calculated.

Why can atomic weights vary slightly?

Natural isotopic composition varies; this tool uses conventional standard atomic weights suitable for classroom calculations.

Sources & review

Equations and examples are checked against the references below. Results are educational and should be independently verified for safety-critical work.

IUPAC periodic table of the elements

Written by the STEM Hub editorial team · Reviewed August 11, 2026 · Review methodology