Avogadro constant

Physico-chemical Adopted SI defining constantExact

$N_A$ = 6.022 140 76 × 10²³ mol⁻¹

Value: 6.022 140 76 × 10²³ mol⁻¹
Standard uncertainty: exact
Relative uncertainty: exact
Status: Exact since the 2019 SI redefinition
Symbol: $N_A$
Source: CODATA 2022 via NIST · NIST entry

Use in a calculation

# Avogadro constant, CODATA 2022 (NIST)
N_A = 6.02214076e23  # mol^-1, exact

# SciPy: scipy.constants.physical_constants['Avogadro constant']

What it is

The number of elementary entities (atoms, molecules, ions…) in one mole. Since 2019 the mole is defined by this exact number rather than as the amount of substance in 12 grams of carbon-12, so the molar mass of carbon-12 is no longer exactly 12 g/mol but is very close to it.

This is one of the seven defining constants of the SI. Its value was measured until the CODATA 2017 special adjustment fixed it, and it has been exact since the new definitions took effect on 20 May 2019.

Equations

$n = N/(N_A)$ amount of substance
$R = N_Ak$ molar gas constant
$F = N_Ae$ Faraday constant

In other units

Avogadro constant per pound-mole and other units →

How the value changed, 1998–2022

Each point is one CODATA adjustment, plotted as its difference from the 2022 value; the bars show one standard uncertainty (an arrowhead means the bar runs off the chart). Diamonds mark adjustments in which the value was exact.

Relative uncertainty

CODATA	Value (uncertainty)	Relative uncertainty	Change from previous
2022	6.022 140 76 × 10²³	exact	exact
2018	6.022 140 76 × 10²³	exact	−1.3 σ of 2014
2014	6.022 140 857(74) × 10²³	1.2 × 10⁻⁸	−1.6 σ of 2010
2010	6.022 141 29(27) × 10²³	4.5 × 10⁻⁸	−1.7 σ of 2006
2006	6.022 141 79(30) × 10²³	5.0 × 10⁻⁸	+0.3 σ of 2002
2002	6.022 1415(10) × 10²³	1.7 × 10⁻⁷	−1.0 σ of 1998
1998	6.022 141 99(47) × 10²³	7.8 × 10⁻⁸

“Change from previous” is the shift in the value divided by the standard uncertainty of the previous adjustment. Earlier values are converted into the current unit where NIST changed it (for example J s to J Hz⁻¹).

Related constants

Physico-chemical exact

R

Gas constant 8.314 462 618… J mol⁻¹ K⁻¹ Physico-chemical exact

F

Faraday constant 96 485.332 12… C mol⁻¹ Physico-chemical

M_{u}

Molar mass constant 1.000 000 001 05(31) × 10⁻³ kg mol⁻¹ Physico-chemical

m_{u}

Atomic mass constant 1.660 539 068 92(52) × 10⁻²⁷ kg Physico-chemical exact

k

Boltzmann constant 1.380 649 × 10⁻²³ J K⁻¹