The Avogadro constant
N_A = 6.02214076 × 10²³ per mole, exact by definition. Since 2019 the mole is simply that many entities, nothing more: the definition no longer mentions carbon-12 or any kilogram.
Updated on October 7, 2026
The value
The Avogadro constant has a unit (per mole); "Avogadro’s number" without a unit is the same digits treated as a pure count. Both are in use, and the exact number is identical.
N_A = 602,214,076,000,000,000,000,000 mol⁻¹
- Value
- 6.02214076 × 10²³
- Unit
- mol⁻¹
- Standard uncertainty
- exact by definition (zero uncertainty)
Source: NIST — CODATA Internationally Recommended Values of the Fundamental Physical Constants (2022)
What the mole actually buys you
A mole is a counting unit, like a dozen, chosen so that the mass of one mole in grams matches the atomic or molecular weight. One mole of carbon-12 weighs 12 g; one mole of water weighs 18.015 g. That is the whole point: it turns a number of atoms, which you cannot weigh out, into a mass, which you can.
Before 2019 N_A was measured, by counting the silicon atoms in a near-perfect sphere, and the definition tied the mole to 12 grams of carbon-12. The revised SI fixed N_A and freed the mole from the kilogram. A side effect: the molar mass of carbon-12 is no longer exactly 12 g/mol, though the difference is far below any uncertainty you will meet in a laboratory.
Source: NIST — CODATA Internationally Recommended Values of the Fundamental Physical Constants (2022)
How to read this page
How to read the numbers here
Every value on "The Avogadro constant" comes from the source named beside its section, never from an estimate of our own with no origin.
Where the data comes from
Each section of "The Avogadro constant" names its official source; the verification date sits at the top of the page.
What the number doesn’t tell you
Atomic weight, molar mass and standard uncertainty are measurement concepts: none of them say anything about a real sample’s purity, physical state or specific isotope.
More about “The Avogadro constant”
N_A among the constants this site tracks
Of the 9 physical constants this module tracks, 7 are exact by definition since the 2019 SI revision — and N_A is one of them. Its value has order of magnitude 1023 in the unit shown on "The Avogadro constant".
The order of magnitude alone says a lot: a value around 1023 needs scientific notation to avoid a row of zeros, which is exactly why physics favours that notation over loose decimals.
Words on this page
Standard uncertainty
The margin attached to a measured value, usually written in parentheses on the last digits. A constant that is "exact by definition" has zero standard uncertainty; a measured one, like the gravitational constant G, always carries some.
Standard definition by BIPM — Bureau International des Poids et Mesures (SI) · Standard uncertainty
Mole
The SI counting unit for atoms, molecules or other entities: one mole is exactly 6.02214076 × 10²³ of them, by the definition fixed in 2019. It is what ties a mass in grams to a number of particles.
Standard definition by BIPM — Bureau International des Poids et Mesures (SI) · Mole
Atomic weight
The average weight of an element’s atoms, weighted by how abundant each isotope is in nature, in atomic mass units. For 14 elements CIAAW publishes a range instead of a single value, because that abundance varies a little from sample to sample.
Standard definition by CIAAW — Commission on Isotopic Abundances and Atomic Weights · Atomic weight
Atomic number
The number of protons in an atom’s nucleus, written Z. It is this number, not the mass, that defines which element an atom is: every atom with 8 protons is oxygen, whether it has 8 or 18 neutrons.
Standard definition by IUPAC · Atomic number
Astronomical unit
The solar system’s reference distance, fixed at exactly 149,597,870,700 metres since 2012, close to the average Earth–Sun distance. It saves writing every planet’s orbit in kilometres with ten digits.
Standard definition by IAU — International Astronomical Union · Astronomical unit