Percent Composition Calculator

The Percent Composition Calculator finds the mass percent of every element in a chemical compound. Type a formula — from simple ones like H2O to compounds with polyatomic ions like Ca(OH)2 or hydrates like CuSO4·5H2O — and the tool returns the molar mass, a full per-element breakdown, and a one-of-a-kind side-by-side comparison of composition by mass versus by number of atoms. That comparison answers the question textbooks rarely make visual: why a compound packed with light hydrogen atoms can still be mostly oxygen by weight.

What Is Percent Composition?

Percent composition (also called percentage composition or mass percent composition) is the percentage by mass that each element contributes to a compound. If you had exactly one mole of the compound, the percent composition tells you how that total mass is divided up among the elements. Because mass is conserved, every element's share is counted exactly once, so the percentages always add up to 100%.

Percent Composition Formula

The mass percent of an element is the mass that element contributes to one mole of compound, divided by the molar mass of the whole compound, times 100:

Here n is the number of atoms of that element in the formula. Do this for every element and the results give the complete percent composition.

How to Calculate Percent Composition (Step by Step)

1. Write the molar mass. Multiply each element's atomic mass by how many atoms of it appear, then add everything together.
2. Find each element's contributed mass. That is the atomic mass times the number of atoms of that element.
3. Divide and multiply by 100. Contributed mass ÷ molar mass × 100 gives the element's mass percent.
4. Check the total. The mass percents should sum to 100% (allowing for tiny rounding).

Worked Example: Water (H₂O)

Water has 2 hydrogen atoms and 1 oxygen atom. Using atomic masses H = 1.008 and O = 15.999:

• Molar mass = (2 × 1.008) + (1 × 15.999) = 18.015 g/mol
• Hydrogen: (2 × 1.008) ÷ 18.015 × 100 = 11.19%
• Oxygen: (1 × 15.999) ÷ 18.015 × 100 = 88.81%

Notice that hydrogen makes up two-thirds of the atoms but only about 11% of the mass — exactly the kind of contrast the dual donut chart is designed to reveal.

Mass Percent vs Number of Atoms

This is the single most common point of confusion in percent composition. The number of atoms and the share of mass are not the same, because elements weigh different amounts. A famous example is glucose:

Half of all the atoms in glucose are hydrogen, yet hydrogen is under 7% of the mass. Oxygen is only a quarter of the atoms but more than half the mass. Counting atoms and weighing them tell two very different stories.

Common Compounds and Their Percent Composition

How to Type Formulas

• Capitalization matters. Co is cobalt, but CO is carbon monoxide. Write each element symbol with one capital letter followed by any lowercase letters.
• Subscripts as plain numbers. Write H2O, C6H12O6, Fe2O3 — no special formatting needed.
• Parentheses for groups. Use them for polyatomic ions, e.g. Ca(OH)2, Fe2(SO4)3, (NH4)2SO4.
• Hydrates. Use a dot, period, or asterisk: CuSO4·5H2O, CuSO4.5H2O, or CuSO4*5H2O all work.

Why Percent Composition Matters

Frequently Asked Questions

What is percent composition?

Percent composition is the percentage by mass of each element in a compound. It tells you what fraction of the compound's total mass comes from each element. For example, water (H₂O) is about 11.19% hydrogen and 88.81% oxygen by mass.

How do you calculate percent composition?

First find the molar mass of the compound by adding the atomic masses of all atoms. Then, for each element, multiply its atomic mass by the number of atoms of that element, divide by the molar mass, and multiply by 100. The result is the mass percent of that element.

What is the formula for mass percent of an element?

Mass percent of an element equals the mass of that element in one mole of the compound divided by the molar mass of the compound, multiplied by 100. In symbols: percent = (n × atomic mass ÷ molar mass) × 100, where n is the number of atoms of the element.

Why is mass percent different from the number of atoms?

Because elements have different atomic masses. A compound can contain many light atoms that still make up only a small share of the total mass. Glucose (C₆H₁₂O₆) has 12 hydrogen atoms but they are only about 6.7% of the mass, while the 6 oxygen atoms are about 53% of the mass.

Can this calculator handle hydrates and parentheses?

Yes. The calculator understands parentheses for polyatomic ions such as Ca(OH)₂ and Fe₂(SO₄)₃, and hydrate dots such as CuSO₄·5H₂O. You can also write hydrates with a period or asterisk, for example CuSO4.5H2O.

Do the percentages always add up to 100%?

Yes. Because every part of the compound's mass belongs to one element, the mass percents of all elements add up to 100% (small rounding differences aside). This calculator shows the total so you can confirm the result is consistent.

Additional Resources

• Mass Fraction (Chemistry) - Wikipedia
• Molar Mass - Wikipedia