# kelvin (K)

The kelvin (abbreviation K), less commonly called the degree Kelvin (symbol, ^{o} K), is the Standard International ( SI ) unit of thermodynamic temperature. One kelvin is formally defined as 1/273.16 (3.6609 x 10 ^{-3} ) of the thermodynamic temperature of the triple point of pure water (H _{2} O).

The kelvin scale differs from the more familiar Celsius or centigrade ( ^{o} C) temperature scale; there is no such thing as a below-zero Kelvin figure. A temperature of 0 K represents absolute zero, the absence of all heat. However, the size of the kelvin "degree" is the same as the size of the Celsius "degree." A change of plus-or-minus 1 ^{o} C is the same as a change of plus-or-minus 1 K.

At standard Earth-atmospheric sea-level pressure, water freezes at 0 ^{o} C or +273.15 K, and boils at +100 ^{o} C or +373.15 K. A temperature of 0 K thus corresponds to -273.15 ^{o} C. A temperature of 273.15 K corresponds to 0 ^{o} C. To convert a kelvin temperature figure to Celsius, subtract 273.15. To convert a Celsius temperature figure to kelvin, add 273.15.

Also see Standard International ( SI ) System of Units.