What is Mole Day?
Mole Day originated in a 1985 article in The Science Teacher magazine. The idea was simple: the date 10/23 (23 October in US notation) maps to the first three digits of Avogadro's number — 6.02 × 10²³ — and the time 6:02 completes the reference.
The National Mole Day Foundation was established in 1991 to promote chemistry education through the annual event. Today thousands of schools in the US, UK, Australia and beyond mark the day with demonstrations, themed food and stoichiometry challenges.
The mole — a quick reminder
The mole (symbol: mol) is the SI unit of amount of substance. One mole of any substance contains exactly 6.022 140 76 × 10²³ entities — whether atoms, molecules, ions or electrons. This number is Avogadro's constant (Nₐ), defined exactly by the 2019 SI revision.
The genius of the mole is bridging scales: the atomic mass of oxygen is 16 unified atomic mass units (u). One mole of oxygen atoms therefore has a mass of exactly 16 grams. You can count atoms by weighing them — using a balance rather than a microscope.
Use our molar mass calculator to calculate the molar mass of any formula and convert between grams and moles in real time.
How to celebrate
- Themed food — guaca-mole, mole-asses biscuits, avoca-mole toast.
- Stoichiometry sprint — convert between grams and moles against the clock.
- Demonstration — weigh out one mole of various household substances (water: 18 g, salt: 58.4 g) to make the number tangible.
- History session — trace the idea of atoms from Dalton (1803) to Avogadro (1811) to Perrin's direct measurement (1908).
- Art challenge — draw the periodic table from memory, marking which elements you know the molar mass of.
The ACS Mole Day page has printable classroom resources, activity guides and videos.
Avogadro's journey to exactness
Amedeo Avogadro (1776–1856) proposed in 1811 that equal volumes of gases at the same temperature and pressure contain equal numbers of particles — a hypothesis ignored for 50 years. It was Jean Perrin who, in 1908, used Brownian motion to calculate the constant experimentally and coined the term "Avogadro's number". Perrin received the 1926 Nobel Prize in Physics for the work.
In 2019 the SI redefinition fixed Avogadro's constant at exactly 6.022 140 76 × 10²³ mol⁻¹ — no more measurement uncertainty. See our physical constants page for the full 2018 CODATA values.