What is Avogadro's number?

Avogadro's number (6.02214076 × 10²³) is the number of constituent particles (atoms, molecules, ions) in one mole of a substance. It was redefined in 2019 as an exact value in the SI system.

What is molar mass and where do I find it?

Molar mass is the mass of one mole of a substance, expressed in g/mol. For elements it equals the atomic mass on the periodic table. For compounds, add the atomic masses of all atoms in the formula.

What does "atoms per formula unit" mean?

For a compound like H₂O, one formula unit contains 3 atoms (2 H + 1 O). If you want the total number of atoms rather than molecules, set this field to the atom count in one formula unit.

Why is the result shown in scientific notation?

Even a tiny sample contains an astronomically large number of atoms — often 10²² or more — so scientific notation is the only practical way to display and compare such values.

Can I calculate molecules instead of atoms?

Yes. Leave "atoms per formula unit" set to 1. The result will then be the number of molecules (or formula units) rather than individual atoms.

How accurate is this calculator?

The calculator uses the 2019 CODATA value of Avogadro's number (6.02214076 × 10²³) and JavaScript's 64-bit floating-point arithmetic, giving about 15 significant figures of precision.

Number of Atoms Calculator

Enter sample mass and molar mass to instantly calculate the number of atoms using Avogadro's number.

Sample Parameters

Sample Mass (g)

Mass of the sample in grams.

Molar Mass (g/mol)

Molar mass of the substance (from the periodic table or formula).

Atoms per Formula Unit

Leave as 1 to count molecules. Use 3 for H₂O (2 H + 1 O), for example.

Quick Examples

Result

Enter values and click Calculate.

Number of Atoms

Step-by-Step Calculation

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Summary

This calculator determines how many atoms are present in a given sample by combining the mole concept with Avogadro's number (6.02214076 × 10²³ atoms/mol). Enter the sample's mass in grams and its molar mass in grams per mole, and the tool computes the result instantly. The full step-by-step derivation is shown so you can follow the chemistry behind the answer.

How it works

Enter the sample mass in grams (e.g., 18 g for water).
Enter the molar mass in g/mol (e.g., 18.015 g/mol for H₂O).
Optionally enter the number of atoms per formula unit (default 1 for monatomic elements).
The tool divides mass by molar mass to get the number of moles.
Moles are multiplied by Avogadro's number (6.02214076 × 10²³) to get atom count.
The result is displayed in scientific notation with a full step-by-step breakdown.

Use cases

Quickly solve stoichiometry homework problems.
Verify hand-calculated atom counts in lab reports.
Understand the scale of matter at the atomic level.
Convert sample masses to molar quantities in chemistry coursework.
Cross-check textbook examples involving Avogadro's number.
Prepare for AP Chemistry or university-level exams.

Frequently Asked Questions

Last updated: 2026-06-04 · Reviewed by Nham Vu