Boyle's Law Definition in Chemistry

Boyle's Law describes the relationship between pressure and volume at constant mass and temperature.
Boyle's Law describes the relationship between pressure and volume of a gas when mass and temperature are held constant. NASA's Glenn Research Center

Boyle's law states that the pressure of an ideal gas increases as its container volume decreases. Chemist and physicist Robert Boyle published the law in 1662. The gas law is sometimes called Mariotte's law or the Boyle-Mariotte law because French physicist Edme Mariotte independently discovered the same law in 1679.

Boyle's Law Equation

Boyle's law is an ideal gas law where at a constant temperature, the volume of an ideal gas is inversely proportional to its absolute pressure. There are a couple of ways of expressing the law as an equation. The most basic one states:

PV = k

where P is pressure, V is volume, and k is a constant. The law may also be used to find the pressure or volume of a system when the temperature is held constant:

PiVi = PfVf


Pi = initial pressure
Vi = initial volume
Pf = final pressure
Vf = final volume

Boyle's Law and Human Breathing

Boyle's law may be applied to explain how people breathe and exhale air. When the diaphragm expands and contracts, lung volume increases and decreases, changing the air pressure inside of them. The pressure difference between the interior of the lungs and the external air produces either inhalation or exhalation.


  • Levine, Ira. N (1978). Physical Chemistry. University of Brooklyn: McGraw-Hill.
  • Tortora, Gerald J. and Dickinson, Bryan. "Pulmonary Ventilation" in Principles of Anatomy and Physiology 11th edition. Hoboken: John Wiley & Sons, Inc., 2006, pp. 863-867.