# Percent Composition Calculator

Free percent composition calculator. Calculate the mass percentage of each element in a chemical compound. Includes common compound presets.

## What this calculates

Calculate the percent composition by mass of each element in a compound. Select from common compounds or enter custom element data. The formula is: % = (atoms × atomic mass) / molecular mass × 100.

## Inputs

- **Select Compound** — options: Custom (enter elements below), Water (H₂O), Carbon Dioxide (CO₂), Sodium Chloride (NaCl), Glucose (C₆H₁₂O₆), Sulfuric Acid (H₂SO₄), Calcium Carbonate (CaCO₃), Ammonia (NH₃), Ethanol (C₂H₆O) — Select a compound or choose custom to enter your own.
- **Element 1: Atomic Mass** (g/mol) — min 0 — Atomic mass of the first element (e.g., C = 12.011).
- **Element 1: Atom Count** — min 0, max 50 — Number of atoms of the first element in the formula.
- **Element 2: Atomic Mass** (g/mol) — min 0 — Atomic mass of the second element.
- **Element 2: Atom Count** — min 0, max 50 — Number of atoms of the second element.
- **Element 3: Atomic Mass** (g/mol) — min 0 — Atomic mass of the third element (0 if none).
- **Element 3: Atom Count** — min 0, max 50 — Number of atoms of the third element.

## Outputs

- **Total Molar Mass** (g/mol) — Total molecular weight of the compound.
- **Percent Composition** — formatted as text — Percent by mass of each element.
- **Calculation** — formatted as text — Step-by-step breakdown.

## Frequently Asked Questions

**Q: What is percent composition?**

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

**Q: How do I calculate percent composition?**

A: For each element: (number of atoms × atomic mass) / molecular mass × 100%. The percentages should sum to 100% (small rounding errors are normal).

**Q: What is percent composition used for?**

A: Percent composition is used to determine empirical formulas from experimental data, verify the purity of compounds, and understand the elemental makeup of substances. It is essential in analytical chemistry and quality control.

**Q: Should the percentages add up to exactly 100%?**

A: Yes, in theory. In practice, small rounding errors may cause the sum to be slightly off (e.g., 99.99% or 100.01%). If your sum is significantly different from 100%, check your calculations.

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Source: https://vastcalc.com/calculators/chemistry/percent-composition
Category: Chemistry
Last updated: 2026-04-21