What is the SI base unit of temperature?

The SI base unit of temperature is the kelvin.

The kelvin, symbol K, is an absolute temperature unit whose scale begins at absolute zero, the theoretical lowest possible temperature. Unlike Celsius and Fahrenheit, it is not written with a degree sign: scientists write 300 K, not 300° K. A change of one kelvin is the same size as a change of one degree Celsius.

The scale honours William Thomson, 1st Baron Kelvin, a 19th-century British physicist who developed the absolute temperature scale. Kelvin temperatures can be converted to Celsius by subtracting 273.15; therefore, 0 °C equals 273.15 K. Absolute zero is 0 K, equivalent to −273.15 °C.

Since 2019, the kelvin has been defined by fixing the numerical value of the Boltzmann constant at exactly 1.380649 × 10⁻²³ joule per kelvin. This definition connects temperature to microscopic particle energy. Celsius is still widely used for everyday weather and cooking, but it is derived from the kelvin rather than being an SI base unit.