Mole Calculator

Last updated: June 2026

Use this mole calculator to convert between mass, moles, particles and number of atoms. It can also estimate molar mass from a chemical formula and count the number of atoms in a sample using Avogadro’s constant.

Key takeaway: The mole connects the mass you weigh in the lab to the number of particles present. The core formulas are n = m / M and N = nNA.

What is a mole?

A mole is a counting unit used in chemistry. One mole of a substance contains Avogadro’s constant number of particles.

NA = 6.02 × 1023 mol−1

The particles may be atoms, molecules or formula units, depending on the substance. For example, one mole of helium contains 6.02 × 1023 helium atoms. One mole of water contains 6.02 × 1023 water molecules. One mole of sodium chloride contains 6.02 × 1023 formula units of NaCl.

Mass to Moles Calculator

To convert mass to moles, use:

n = m / M

where n is the amount of substance in mol, m is the mass in grams, and M is the molar mass in g mol−1.

For example, 18.0 g of H2O has a molar mass of about 18.0 g mol−1, so it contains about 1.00 mol of H2O.

Moles to Mass Calculator

To convert moles to mass, rearrange the mole formula:

m = nM

If you know the amount of substance and the molar mass, multiply them to find the mass.

Moles to Particles Calculator

To convert moles to particles, use:

N = nNA

where N is the number of particles and NA is Avogadro’s constant.

Particles to Moles Calculator

To convert particles to moles, rearrange the Avogadro constant formula:

n = N / NA

This is useful when the question gives a number of atoms, molecules or formula units directly.

Atom Count Calculator

An atom count calculator finds the total number of atoms in a sample. First calculate the number of particles of the substance. Then multiply by the number of atoms per formula unit.

total number of atoms = number of particles × atoms per formula unit

For water:

H2O has 3 atoms per molecule.

Therefore, one mole of H2O contains:

3 × 6.02 × 1023 atoms

If you want to count only hydrogen atoms in water, multiply by 2 instead, because each H2O molecule contains 2 hydrogen atoms.

Worked example: number of hydrogen atoms in water

Find the number of hydrogen atoms in 18.0 g of water.

First calculate the amount of water:

n = m / M = 18.0 / 18.0 = 1.00 mol

Number of water molecules:

N = nNA = 1.00 × 6.02 × 1023

Each water molecule contains 2 hydrogen atoms, so:

number of H atoms = 2 × 6.02 × 1023 = 1.20 × 1024

Common exam traps

Trap 1: Do not confuse moles with molecules. One mole is a very large number of particles.
Trap 2: When counting atoms in a compound, remember to multiply by the number of atoms in each formula unit.
Trap 3: H2O has 3 atoms per molecule, but only 2 hydrogen atoms per molecule.
Trap 4: For ionic compounds such as NaCl, the particle counted by N = nNA is usually the formula unit, not a single molecule.
Trap 5: Use molar mass in g mol−1 when the mass is in grams.

Frequently Asked Questions

What does a mole calculator calculate?

A mole calculator converts between mass, moles and particles. This page also counts total atoms or atoms of a specific element in a sample.

What is Avogadro’s constant?

Avogadro’s constant is the number of particles in one mole of substance. It is approximately 6.02 × 1023 mol−1.

How do I calculate the number of atoms in a compound?

First calculate the number of molecules or formula units. Then multiply by the number of atoms in each formula unit.

Why is Atom Count Calculator inside Mole Calculator?

Atom count questions use the mole concept and Avogadro’s constant. That is why they fit naturally inside the Mole Calculator page.

Can I override the molar mass?

Yes. If your exam question gives a specific molar mass or relative formula mass, enter it in the molar mass override box.

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