How to use the Avogadro's Number
- Enter the amount in moles to obtain the number of particles.
- Specify whether the counted entities are atoms, molecules or ions.
- For grams, first divide mass by molar mass using the worked example below; the widget only takes moles.
- For atoms to molecules, use the molecular formula to determine the correct atom-to-molecule ratio.
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 converts grams to moles to particle count in one step, which is the conversion most grams-to-moles and atoms-to-molecules 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
Two common conversions: how many moles and molecules are in 18.5 g of glucose, C₆H₁₂O₆ (molar mass 180.16 g/mol), and how many molecules are in 3.5 × 10²³ atoms of oxygen gas (O₂)?
- Part 1, grams to moles: moles = 18.5 / 180.16 = 0.1027 mol.
- Part 1, moles to molecules: 0.1027 × 6.022e23 = 6.18e22 molecules.
- Part 1, molecule to atoms: 24 × 6.18e22 = 1.48e24 total atoms.
- Part 2, atoms to molecules: each O₂ molecule holds 2 atoms, so molecules = 3.5e23 / 2 = 1.75e23.
- Part 2, check by moles: 3.5e23 / 6.022e23 = 0.581 mol of atoms, which is 0.291 mol of O₂, matching 1.75e23 molecules.
18.5 g of glucose is 0.1027 moles, about 6.18 × 10²² molecules and 1.48 × 10²⁴ atoms; 3.5 × 10²³ oxygen atoms form 1.75 × 10²³ O₂ molecules.
Frequently asked questions
How do I convert grams to moles?+
Divide the mass in grams by the molar mass in g/mol. For water, 36 g ÷ 18.02 g/mol = 2.0 mol. Divide first, then enter the resulting moles in the widget to calculate the particle count.
How do I convert atoms to molecules?+
For a count of all atoms, divide by the total number of atoms in one molecule. For atoms of a specific element, divide by that element’s subscript. For O₂, 3.5 × 10²³ atoms ÷ 2 = 1.75 × 10²³ molecules.
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 element’s subscript to count that element, or by the sum of subscripts to count all atoms. Reverse using the same ratio.
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.