In simple terms
A mole is a counting unit for very small things. One mole is a fixed number of particles, the way a dozen is a fixed number of eggs.
The number is enormous because atoms are tiny. NIST calls the mole a shorthand way to talk about huge numbers, especially of tiny things.
How it works
The current definition reads: the mole, symbol mol, is the SI unit of amount of substance, and one mole contains exactly 6.02214076 times 10 to the power 23 elementary entities.
An elementary entity may be an atom, a molecule, an ion, an electron, any other particle or a specified group of particles. You always have to say what you are counting.
That number is the Avogadro constant, now fixed by definition rather than measured. The 26th General Conference on Weights and Measures adopted the change in November 2018, and it took effect with the 2019 revision of the SI.
The older 1971 definition tied the mole to carbon 12, by setting the molar mass of carbon 12 at exactly 0.012 kilograms per mole. That made the mole depend on how the kilogram was defined. Fixing the Avogadro constant broke the link and drew a cleaner line between amount of substance and mass.
Why it matters
Chemistry is done by counting particles, but a balance can only weigh. The mole converts between the two, so a recipe written in particles can be carried out in grams.
Concentration works the same way. The SI unit is the mole per cubic meter, though moles per liter is the everyday working unit in laboratories.
Where you’ll see it
- Chemical equations and laboratory calculations
- Reagent and drug concentrations on labels
- Blood test results reported in millimoles per liter
- Industrial process and emissions accounting
Example
A method calling for 0.5 mol of a reagent is asking for half of 6.02214076 times 10 to the power 23 units of it, whatever that quantity happens to weigh.
Often confused with
Amount of substance is not mass. The mole counts entities. The kilogram measures how much those entities weigh, and the 2019 revision was made partly to keep the two ideas apart.
Key facts
- One mole contains exactly 6.02214076 times 10 to the power 23 elementary entities, which fixes the numerical value of the Avogadro constant.1
- An elementary entity may be an atom, a molecule, an ion, an electron, any other particle or a specified group of particles.1
- The definition was adopted at the 26th General Conference on Weights and Measures in November 2018 and took effect with the 2019 SI revision.1
- The previous 1971 definition set the molar mass of carbon 12 at exactly 0.012 kilograms per mole, which tied the mole to the kilogram.3
- Amount of substance carries the symbol n, and the SI unit of concentration is the mole per cubic meter.2
Related concepts
Quick checkSince 2019, what fixes the size of a mole?Show answer
A fixed numerical value of the Avogadro constant, not a mass of carbon 12.
Sources
- Bureau International des Poids et Mesures. SI base unit: mole (mol). Undated (accessed 16 September 2026)
- National Institute of Standards and Technology. SI Units: Amount of Substance. 11 August 2026 (accessed 16 September 2026)
- Bureau International des Poids et Mesures. History of the SI: mole. Undated (accessed 16 September 2026)
Editorially reviewed by Specialty Digest Editorial TeamLast reviewed September 16, 2026Researched and drafted with AI assistanceReport an issue