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 Fahrenheit?

Fahrenheit is a temperature scale named after Daniel Gabriel Fahrenheit (1686–1736), the German physicist who proposed it in 1724.

In this scale, the freezing point of water is 32 degrees Fahrenheit (written "32 °F"), the boiling point is 212 degrees, placing the boiling and freezing points of water exactly 180 degrees apart. On the Celsius scale, the freezing and boiling points of water are exactly 100 degrees apart, thus the unit of this scale, a degree Fahrenheit, is ⁵⁄₉ of a degree Celsius. The Fahrenheit scale coincides with the Celsius scale at -40 °F, which is the same temperature as -40 °C.

Absolute zero is −459.67 °F. The Rankine temperature scale was invented to use degrees the same size as Fahrenheit degrees, but with absolute zero equal to 0 °R, or −459.67 °F.