Atmospheric pressure is measured using a barometer; other gas pressures can be measured using one of several types of manometers The pressure of a gas may be expressed in the SI unit of pascal or kilopascal, as well as in many other units including torr, atmosphere, and bar.

The pressure of a given amount of gas is directly proportional to its absolute temperature, provided that the volume does not change (Amontons’s law). The volume of a given gas sample is directly proportional to its absolute temperature at constant pressure (Charles’s law).

Amontons’s law — (also, Gay-Lussac’s law) pressure of a given number of moles of gas is directly proportional to its kelvin temperature when the volume is held constant. Boyle’s law — volume of a given number of moles of gas held at constant temperature is inversely proportional to the pressure under which it is measured. Charles’s law — volume of a given number of moles of gas is directly proportional to its kelvin temperature when the pressure is held constant. Avogadro’s law — volume of a gas at constant temperature and pressure is proportional to the number of gas molecules.