Avogadro's law, a gas law discovered by Amedeo Avogadro, dictates that the number of molecules in a specific volume of gas is dependent on its pressure and temperature; and is independent of the size or mass of the gas molecules.

The law has three facets:

1. The amount of moles is directly proportional to the volume of the gas.
2. The amount of moles is directly proportional to its pressure.
3. The amount of moles is inversely proportional to its temperature.

Multiply all that by a constant to make an equality instead of a proportion and you get:

<math>n = \frac{PV}{RT}<math>

With pressure expressed in kPa, volume in Liters, and temperature in Kelvins; R (the molar gas constant) is equal to 8.21 (kPa × L)/(K × mol), and n represents the number of moles. This is also known as the ideal gas law: PV=nRT.

One mole of any gas occupies approximately 22.4 Liters (dm³) at STP.

The number of molecules in one mole is called Avogadro's number: approximately 6.022 × 10²³ particles/mole.

fr:Loi d'Avogadro ja:アボガドロの法則 pl:Prawo Avogadra ru:Закон Авогадро