the Kelvin scale
/ KEL-vin /
The Kelvin scale is the temperature scale physics prefers, because it starts at the coldest temperature that can possibly exist. Its zero point, 0 K, is absolute zero, so there are no negative temperatures on this scale. That makes it the natural scale for the laws of thermodynamics, where many formulas simply do not work with the negative numbers that Celsius or Fahrenheit allow.
One kelvin is exactly the same size as one Celsius degree, so the two scales differ only in where they put their zero. The conversion is T(K) = T(C) + 273.15. Absolute zero is 0 K = -273.15 C; water freezes at 273.15 K (0 C) and boils at 373.15 K (100 C) at standard pressure. The kelvin is one of the seven SI base units, and it is now defined by fixing the Boltzmann constant, tying temperature directly to energy.
A small but real convention: we write '300 K' and say 'three hundred kelvin,' never 'degrees kelvin,' and the symbol K carries no degree sign. The scale is named after William Thomson, Lord Kelvin, who proposed an absolute temperature scale in 1848.
A comfortable room temperature of about 20 C is 293 K; a summer day at 30 C is 303 K. To convert, just add 273.15 to the Celsius reading.
Kelvin and Celsius have the same degree size; only their zero points differ, by 273.15.
Write 'kelvin' and 'K' with no degree symbol and never say 'degrees kelvin'; and remember absolute zero (0 K) can be approached but never actually reached.