How to use the Avogadro's Number
- Choose what you know: moles, particles or mass.
- Enter the molar mass in g/mol if you are working with mass.
- Enter your known quantity.
- Read the other two quantities directly.
How the calculation works
One mole contains exactly 6.02214076 × 10²³ entities — a value fixed by definition since the 2019 SI redefinition, rather than measured. Particles = moles × Nₐ, and moles = mass ÷ molar mass. Chaining the two gives mass to particle count in one step, which is the conversion most stoichiometry problems actually need.
The unit of the 'entity' must be stated. A mole of water molecules contains 6.022 × 10²³ molecules but three times as many atoms, and a mole of NaCl formula units yields two moles of ions in solution. Molar mass is read off the periodic table as the sum of atomic masses in the formula, which is why it is numerically equal to the formula mass in atomic mass units.
N = n × 6.02214076e23 ; n = mass / molar massSource: SI definition of the mole (2019 redefinition): Avogadro constant fixed at 6.02214076 × 10²³ mol⁻¹.
Worked example
How many molecules are in 18.5 g of glucose, C₆H₁₂O₆ (molar mass 180.16 g/mol)?
- Moles = 18.5 / 180.16 = 0.1027 mol.
- Molecules = 0.1027 × 6.022e23 = 6.18e22.
- Carbon atoms = 6 × 6.18e22 = 3.71e23.
- Total atoms = 24 × 6.18e22 = 1.48e24.
About 6.18 × 10²² glucose molecules, containing roughly 1.48 × 10²⁴ atoms in total.
Frequently asked questions
Why 6.022 × 10²³ specifically?+
It was chosen so that one mole of carbon-12 weighs 12 grams, and is now fixed exactly by SI definition.
Is molar mass the same as molecular weight?+
Numerically yes for a given formula, but molar mass carries units of g/mol while molecular weight is dimensionless.
Do I count molecules or atoms?+
Whichever the question asks. Multiply molecules by the subscript to get atoms of a given element.
Does the constant apply to ions and electrons?+
Yes. A mole counts any specified entity, including ions, electrons or photons.
Last reviewed August 31, 2026. We review this page whenever the underlying formula, tax year, published rate or standard changes.