Avogadro's Number Calculator

Avogadro's number (N_A = 6.02214076 × 10²³ mol⁻¹) is defined so that 1 mole of carbon-12 atoms has a mass of exactly 12 grams. The 2019 SI redefinition fixed N_A to an exact value. It is large because atoms are extremely small: a single carbon atom weighs about 2 × 10⁻²³ g.

First convert mass to moles: moles = mass / molar mass. Then multiply by Avogadro's number: N = moles × 6.022 × 10²³. For example, 12 g of carbon = 1 mol, so it contains 6.022 × 10²³ carbon atoms. For compounds, this gives molecules; for ionic compounds, formula units.

Divide the molar mass by Avogadro's number. A single water molecule: 18.015 g/mol ÷ (6.022 × 10²³) = 2.99 × 10⁻²³ g. A single iron atom: 55.845 ÷ (6.022 × 10²³) = 9.27 × 10⁻²³ g. These masses are also expressible in atomic mass units (amu or Da), where 1 amu = 1.66054 × 10⁻²⁴ g.

They are three ways to count the particles in a mole, matching the particle-type selector. Atoms are single elements: one mole of carbon = 6.022 × 10²³ atoms. Molecules are groups of atoms held by covalent bonds: one mole of water = 6.022 × 10²³ H₂O molecules, but three times as many atoms. Formula units are the smallest whole-number ratio in an ionic compound such as NaCl, which has no discrete molecules: one mole of NaCl = 6.022 × 10²³ formula units, each releasing one Na⁺ and one Cl⁻ ion. Avogadro's number counts whichever entity you select.