\[0^{\circ} \: Celsius\]

is defined to be the freezing point of water at standard pressure \[p_0=1.1325 \times 10^5 N/m^2\]

\[100^{\circ} \: Celsius\]

is defined to be the boiling point of water at standard pressure \[p_0=1.1325 \times 10^5 N/m^2\]

The Kelvin scale is more fundamental. An increase of

\[1^{\circ} Celsius\]

is the same increase as an increase of \[1^{\circ} Kelvin\]

, but the zero of the Kelvin scale is defined as the temperature at which no more energy can be extracted from a system, and is equal to \[-273.16^{\circ} Celsius\]

.When doing calculations, typically we use the Kelvin scale eg when using the ideal gas equation of state

\[1 pV=nRT\]

.