Last modified on October 6, 2019, at 17:54

Absolute temperature scale

The term absolute temperature scale usually refers to the Kelvin temperature scale, but can also refer to the Rankine scale. These scales are based on the conventional scales for measuring temperature – Celsius in most non-US countries and Fahrenheit in the United States.

Need for an absolute scale

Chemists and physicists working with gases came to the realization that the conventional temperature scales were not adequate for characterizing the effect of temperature on the decrease in volume and pressure of gases with the corresponding decrease in the actual temperature of the gases.

Ultimately, they realized that if they could decrease the temperature of an ideal gas to a point where all molecular motion ceased, that would be a good "absolute zero" point. They found that this temperature would be -273.15 °C (-459.67 °F).

Kelvin scale

The Kelvin scale, which is based on the Celsius scale, sets the absolute zero point at 0 Kelvin (or just 0K).

Thus, the melting point of water would be 273.15 K (0 °C + 273.15).[1] Likewise, the boiling point of water at pressure 1 atm is 373.15 K (100 °C + 273.15) and the triple point of water is at 273.16 K.

Rankine scale

Like the Kelvin scale, the absolute zero temperature on the Rankine scale is at its zero point - 0°R. However, one degree on the Rankine scale is one degree on the Fahrenheit scale.

Thus, the freezing point of water would be 491.67 °R (32 °F + 459.67), and the boiling point of water would be 671.67 °R (212 °F + 459.67).


  1. Kelvin (