The volume of a fixed mass of gas at constant pressure expands by a constant fraction of its volume at 0°C for each Celsius degree or kelvin its temperature is raised.

For any ideal gas the fraction is approximately 1/273. This can be expressed by the equation V = Vo (1 + t/273), where Vo is the volume at 0°C and V is its volume at t°C. This is equivalent to the statement that the volume of a fixed mass of gas at constant pressure is proportional to its thermodynamic temperature, V = kT, where k is constant. The law is also known as Gay-Lussac's law.

An equation similar to that given above applies to pressures for ideal gases p = po (1 + t/273), a relationship known as Charle's law of pressures.