Last updated: 2026-06-14

Understanding the mole

The mole is chemistry's counting unit. One mole is exactly 6.02214076 × 10²³ particles — atoms, molecules, ions, or formula units — a quantity known as Avogadro's number. Because a mole of any substance has a mass in grams numerically equal to its molar mass, the mole lets chemists count particles simply by weighing a sample.

The mole formula

Rearranged, the same relation gives:

• Mass: $m = n \times M$
• Molar mass: $M = m \div n$

Once you know the moles, the number of individual particles follows from Avogadro's number:

Worked example

How many moles are in 36.03 g of water (H₂O)?

The molar mass of water is 2(1.008) + 16.00 = 18.015 g/mol, so:

That is 2 × 6.022 × 10²³ = 1.204 × 10²⁴ water molecules.

Finding molar mass

The molar mass of a compound is the sum of the atomic masses of all its atoms, read from the periodic table:

Enter the molar mass you calculate for your compound, and this tool handles the conversion to or from moles.

Molar mass vs. molecular weight

Molar mass and molecular weight are numerically identical but expressed in different units. Molecular weight is the mass of one molecule in atomic mass units (u); molar mass is the mass of one mole in grams per mole (g/mol). Water is 18.015 u per molecule and 18.015 g/mol — the same number, two perspectives on the same quantity.

Why the mole matters

Chemical reactions combine particles in whole-number ratios, not in equal masses. A balanced equation such as 2H₂ + O₂ → 2H₂O means two moles of hydrogen react with one mole of oxygen. Converting masses to moles is therefore the first step in nearly every stoichiometry problem, which is why the mole calculator is one of the most-used tools in chemistry.