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An amagat is a practical unit of number density. Although it can be applied to any substance at any conditions, it is defined as the number of ideal gas molecules per unit volume at 1 atm (= 101.325 kPa) and 0 °C (= 273.15 K).[1] It is named after Emile Amagat, who also has Amagat's law named after him. The abbreviated form of amagat is "amg."


Number density in amg, denoted here by \eta, is defined as

\eta= \frac{n}{n_0},

where n0 = 1 amg = 2.686 7805×1025 m−3 = 44.615 036 mol/m3 is the Loschmidt constant.

In practice, number density of an ideal gas at pressure P and temperature T can be calculated as[2]

\eta= \left(\frac{p}{p_0}\right)\left(\frac{T_0}{T}\right)\, {\rm amg},

where T0 = 273.15 K and p0 = 101.325 kPa.


Number density of an ideal gas (such as air) at room temperature (20 °C) and 1 atm (101.325 kPa) is

\eta= \left(\frac{1\, {\rm atm}}{p_0}\right)\left(\frac{273.15\, {\rm K}}{(273.15+20)\, {\rm K}}\right) {\rm amg}=0.932\, {\rm amg}.


  1. ^ Hirschfelder, Joseph O.; Curtiss, Charles F.; Bird, R. Byron (1967), Molecular Theory of Gases and Liquids (Corrected printing ed.), John Wiley & Sons, Inc. 
  2. ^ In this formula, absolute units of pressure and temperature, relative to vacuum and absolute zero, must be used.