Kelvin (K)

How to read it

OpenStax’s Chemistry 2e gives the IUPAC convention: kelvin, all lowercase, for the word, K for the symbol, and neither the word “degree” nor the degree sign. The degree Celsius keeps both. On the two scales water freezes at 273.15 K (0 °C) and boils at 373.15 K (100 °C), and normal body temperature is about 310 K (37 °C). NIST’s Guide for the Use of the SI calls the degree Celsius a special name for the kelvin, used for Celsius temperatures: the two are equal in size, so a temperature difference has the same number in either. Since 2019 the BIPM’s SI Brochure has defined the kelvin by fixing the Boltzmann constant k at exactly 1.380 649 × 10⁻²³ J K⁻¹.

Also searched as: degrees Kelvin, °K, K, kelvin symbol, K in physics, K temperature.

Look-alikes

Unicode has a separate KELVIN SIGN, K (U+212A), but its chart marks it as the same as the ordinary capital K (U+004B). The lowercase k is the prefix kilo, a thousand, as in km and kg (BIPM). The °C and °F on a multimeter’s dial are degrees and keep the sign.