The kelvin (symbol: K) is a unit increment of temperature and is one of the seven SI base units. The Kelvin scale is a thermodynamic (absolute) temperature scale where absolute zero, the theoretical absence of all thermal energy, is zero (0 K).

The Kelvin scale and the kelvin are named after the British physicist and engineer William Thomson, 1st Baron Kelvin (1824-1907), who wrote of the need for an “absolute thermometric scale”.

The kelvin unit and its scale, by international agreement, are defined by two points: absolute zero, and the triple point of Vienna Standard Mean Ocean Water (VSMOW). This definition also precisely relates the Kelvin scale to the Celsius scale. Absolute zero—the temperature at which nothing could be colder and no heat energy remains in a substance—is defined as being precisely 0 K and −273.15 °C. The triple point of water is defined as being precisely 273.16 K and 0.01 °C. This definition does three things:

1. It fixes the magnitude of the kelvin unit as being precisely 1 part in 273.16 parts the difference between absolute zero and the triple point of water;
2. It establishes that one kelvin has precisely the same magnitude as a one-degree increment on the Celsius scale; and
3. It establishes the difference between the two scales’ null points as being precisely 273.15 kelvins (0 K = −273.15 °C and 273.16 K = 0.01 °C). Temperatures in kelvin can be converted to other units per the table at top right.

What is a Rankine?

Rankine is a thermodynamic (absolute) temperature scale named after the Scottish engineer and physicist William John Macquorn Rankine, who proposed it in 1859.

The symbol is R (or Ra if necessary to distinguish it from the Rømer and Réaumur scales). As with the Kelvin scale (symbol: K), zero on the Rankine scale is absolute zero, but the Rankine degree is defined as equal to one degree Fahrenheit, rather than the one degree Celsius used by the Kelvin scale. A temperature of -459.67 °F is exactly equal to 0 R.

A few engineering fields in the U.S. measure thermodynamic temperature using the Rankine scale. However, throughout the scientific world where measurements are made in SI units, thermodynamic temperature is measured in kelvin.