Kelvin (K) is the base unit of temperature in the International System of Units (SI). It is named after the Scottish physicist William Thomson, 1st Baron Kelvin, who made major contributions to thermodynamics. The Kelvin scale begins at absolute zero—the theoretical point at which all molecular motion stops. For this reason, temperatures in Kelvin are written simply as “K,” not degrees (°K).
The Kelvin scale uses the same size increments as Celsius, but its zero point is absolute zero rather than the freezing point of water. This makes it especially suitable for scientific work, as it avoids negative values and allows temperature to be directly proportional to thermodynamic properties. On this scale, water freezes at 273.15 K.
Kelvin is widely used in physics, chemistry, and cosmology, particularly in precise thermodynamic measurements and calculations.