Updated September 21, 2026

By FreeCalz

Percent Composition Calculator – Calculate Elemental Percent by Mass | FreeCalz

Percent Composition Calculator

Calculate the percentage by mass of each element in a compound from its chemical formula. Find molar mass, elemental mass contributions, and step-by-step percent composition.

Calculate Percent Composition by Mass

Enter a chemical formula such as H2O, CO2, Ca(OH)2, or Al2(SO4)3. The calculator parses the formula, calculates molar mass, and determines the mass percentage of every element.

Formula: Percent by mass = (mass contributed by an element ÷ molar mass of the compound) × 100

Chemical Formula Use element symbols, subscripts, and parentheses. Examples: H2O, NaCl, Ca(OH)2, Al2(SO4)3.

Sample Mass (Optional) Optional. Enter a sample mass to show the actual mass of each element in that sample.

Percent Composition Result

—Molar Mass (g/mol)

—Number of Elements

—Total Atoms

—Percent Total

Element	Atoms	Atomic Mass	Mass Contribution	Percent by Mass	Sample Mass

Step-by-Step Calculation

Step 1 — Parse the chemical formula—

Step 2 — Calculate molar mass—

Step 3 — Calculate each elemental percentage—

Step 4 — Verify the total—

What Is Percent Composition?

Percent composition by mass describes what percentage of a compound’s total mass is contributed by each element. It is calculated from the atomic masses and the number of atoms of each element represented in the chemical formula.

For a pure compound, the elemental mass percentages add to approximately 100%. Small differences from exactly 100% can result from rounding atomic masses and displayed percentages.

Percent Composition Formula

For each element: % by mass = (elemental mass contribution ÷ molar mass) × 100 Elemental mass contribution: number of atoms × atomic mass Molar mass: sum of all elemental mass contributions

The same method can be used whether the compound contains two elements or many elements. Parentheses in formulas must be interpreted correctly because a subscript outside a group multiplies every element inside that group.

How to Calculate Percent Composition

Write the chemical formula of the compound.
Count the atoms of each element in the formula.
Multiply each element’s atom count by its atomic mass.
Add all elemental contributions to obtain the molar mass.
Divide each elemental contribution by the total molar mass.
Multiply each result by 100 to obtain the percent by mass.
Check that the elemental percentages total approximately 100%.

Worked Example: Percent Composition of Water

Formula: H2O

Hydrogen contribution: 2 × 1.008 = 2.016 g/mol

Oxygen contribution: 1 × 15.999 = 15.999 g/mol

Molar mass: 2.016 + 15.999 = 18.015 g/mol

Hydrogen: (2.016 ÷ 18.015) × 100 ≈ 11.19%

Oxygen: (15.999 ÷ 18.015) × 100 ≈ 88.81%

Parentheses in Chemical Formulas

Parentheses indicate that the subscript following the closing parenthesis applies to the entire group. For example, Ca(OH)2 contains one calcium atom, two oxygen atoms, and two hydrogen atoms.

Similarly, Al2(SO4)3 contains two aluminum atoms, three sulfur atoms, and twelve oxygen atoms. Correctly interpreting grouped formulas is essential when calculating molar mass and percent composition.

Percent Composition and Empirical Formula

Percent composition can be used as a starting point for determining an empirical formula from experimental elemental percentages. In that process, each percentage is converted to a relative amount of substance by dividing by the corresponding atomic mass, and the resulting mole ratios are normalized.

Percent composition itself does not determine a molecular formula unless additional information, such as molar mass, is available.

Common Mistakes

Forgetting subscripts in the chemical formula.
Applying a parenthesis subscript to only one element instead of the entire group.
Using the atomic number instead of atomic mass.
Dividing by an elemental contribution instead of the total molar mass.
Rounding too early in the calculation.
Expecting the displayed percentages to total exactly 100.000% when rounding is applied.

Accuracy, Assumptions & Limitations

The calculator uses representative standard atomic-weight values and treats the supplied formula as a neutral chemical composition. Results are mathematical composition calculations; they do not determine isotope-specific composition, hydration state, purity, or experimental uncertainty unless those factors are explicitly represented in the formula or input.

Atomic-weight conventions can vary slightly between reference sources, so results may differ by a small amount in the last displayed digits.

Frequently Asked Questions

What is percent composition by mass?

It is the percentage of a compound’s total mass contributed by a particular element.

How do I calculate percent composition?

Calculate each element’s mass contribution, divide it by the compound’s total molar mass, and multiply by 100.

Does percent composition add up to 100%?

For a pure compound, the elemental percentages should total approximately 100%, with small differences possible because of rounding.

Can I enter formulas with parentheses?

Yes. The calculator supports grouped formulas such as Ca(OH)2 and Al2(SO4)3.

What atomic masses does the calculator use?

It uses representative standard atomic-weight values for the supported elements. The displayed result may vary slightly from a reference using a different atomic-weight convention.

Can percent composition determine an empirical formula?

Percent composition can be converted into relative mole amounts and normalized to help determine an empirical formula, but the percent-composition calculation itself is a separate step.

Can I calculate the actual mass of an element in a sample?

Yes. Enter an optional sample mass. The calculator multiplies the elemental mass fraction by the sample mass.

What is the difference between percent composition and mass percent?

In this context they refer to the same basic quantity: the percentage of total compound mass attributable to a specified element.

References and Data Sources

IUPAC Periodic Table of the Elements

Authoritative periodic-table information and standard atomic-weight context.

PubChem Periodic Table

Element symbols, atomic properties, and chemistry reference information from NCBI.

NIST Atomic Weights and Isotopic Compositions

NIST reference information for atomic weights and isotopic compositions.

NIST Chemistry WebBook

NIST Standard Reference Database containing chemical and physical property information.

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Disclaimer

This calculator is provided for general educational and informational purposes. Results depend on the chemical formula and atomic-weight values used. For research, laboratory, regulatory, or other applications where accuracy is important, verify the calculation against an appropriate authoritative reference and the relevant experimental or professional requirements.