What Is Kelvin (K)?
Definition
Kelvin (K): The SI base unit of thermodynamic temperature, symbol K. The scale starts at absolute zero, and an interval of one kelvin equals an interval of one degree Celsius.
The kelvin (K) is the SI base unit of thermodynamic temperature. The kelvin scale starts at absolute zero, the lowest temperature possible in theory: 0 K is −273.15 °C. A difference of one kelvin equals a difference of one degree Celsius. Since 2019 the kelvin has been defined by fixing the value of the Boltzmann constant.
Where it appears in the formulas
T(K) = t(°C) + 273.15. The temperature converter takes this definition from the BIPM SI Brochure, uses t(°F) = 1.8 × t(°C) + 32 between Celsius and Fahrenheit, and rejects values below absolute zero. The Boltzmann constant is fixed at k = 1.380 649 × 10⁻²³ J/K.
No degree sign
The kelvin takes no degree sign: write “300 K”, not “300 °K”. The unit is the kelvin, not the “degree Kelvin”.