The Loschmidt constant or Loschmidt's number (symbol: n₀) is the number of particles (atoms or molecules) of an ideal gas in a given volume (the number density), and usually quoted at standard temperature and pressure. The 2014 CODATA recommended value is 2.6867811(15)×10²⁵ per cubic metre at 0 °C and 1 atm and the 2006 CODATA recommended value was 2.686 7774(47)×10²⁵ per cubic metre at 0 °C and 1 atm. It is named after the Austrian physicist Johann Josef Loschmidt, who was the first to estimate the physical size of molecules in 1865. The term "Loschmidt constant" is also sometimes used to refer to the Avogadro constant, particularly in German texts.

The Loschmidt constant is given by the relationship:

${\displaystyle n_{0}={\frac {p_{0}}{k_{\rm {B}}T_{0}}}}$

where p₀ is the pressure, k_B is the Boltzmann constant and T₀ is the thermodynamic temperature. It is related to the Avogadro constant, N_A, by:

${\displaystyle n_{0}={\frac {p_{0}N_{\rm {A}}}{RT_{0}}}}$

where R is the gas constant.

Being a measure of number density, the Loschmidt constant is used to define the amagat, a practical unit of number density for gases and other substances:

1 amagat = n₀ = 2.6867811×10²⁵ m⁻³,

such that the Loschmidt constant is exactly 1 amagat.