Mole Day 2026 Is Friday, Oct. 23, From 6:02 a.m. to 6:02 p.m.: What It Celebrates and a Free Practice Sheet

Mole Day 2026 is Friday, Oct. 23, and runs from 6:02 a.m. to 6:02 p.m. The date and time spell out 6.02 x 10^23, the first digits of the Avogadro constant, the number of particles in one mole. Chemistry classes mark it with problems, puns and treats. Below: what the number means, how the day started, and a free practice sheet with an answer key.

Why 6:02 on Oct. 23

Write the Avogadro constant in short form, 6.02 x 10^23, and the pieces line up with the clock and the calendar: 6:02 on the 23rd day of the 10th month. The National Mole Day Foundation asks schools to celebrate from 6:02 a.m. to 6:02 p.m. local time. In 2026 that is a school day, a Friday.

What a mole is

The mole is one of the seven base units of the International System of Units (SI), the unit for an amount of substance. One mole contains exactly 6.022 140 76 x 10^23 elementary entities, such as atoms, molecules, ions or electrons, the International Bureau of Weights and Measures says. That number has been exact since the SI was revised in 2019; before then the mole was defined through the mass of carbon-12, and the constant was a measured value. The National Institute of Standards and Technology lists it as exact in its table of constants.

The mole links the tiny scale of atoms to amounts you can weigh. One mole of water molecules weighs about 18.015 grams, a little over a tablespoon of water.

How Mole Day started

Maurice Oehler, a high school chemistry teacher from Prairie du Chien, Wisconsin, founded the National Mole Day Foundation on May 15, 1991, the foundation says, after an article in The Science Teacher in the early 1980s suggested the idea. The foundation picks a pun-based theme each year; its 2025 theme was “MOLEassic Park.” It had not posted a 2026 theme on its site when we checked on Oct. 21.

Try it: four mole problems

These use standard atomic weights from the International Union of Pure and Applied Chemistry (IUPAC): hydrogen 1.008, carbon 12.011, oxygen 15.999, sodium 22.990, chlorine 35.45.

1. How many water molecules are in 18.015 grams of water?
2. How many moles are in 44.009 grams of carbon dioxide?
3. How many atoms are in 2 moles of helium?
4. What is the mass of 0.5 mole of sodium chloride (table salt)?

Answers: 1. One mole, or 6.022 x 10^23 molecules. 2. One mole, since carbon dioxide’s molar mass is 12.011 + 2 x 15.999 = 44.009 grams per mole. 3. 1.204 x 10^24 atoms. 4. 29.22 grams, half of the 58.44-gram molar mass.

Print the practice sheet

Our free one-page sheet has the constant and the definition, the four problems with space to work, and the answer key at the bottom so it can be folded under. Mark the day on our October 2026 printable calendar or a printable school calendar. For everything else on Oct. 22 and the week ahead, see October 22, 2026: this day in history; students planning test dates can check the SAT and ACT 2026-27 dates.