Kelvin

Kelvin (symbol K, not °K) is the SI base unit of temperature. The zero point is absolute zero — the theoretical minimum temperature at which all molecular motion ceases — equal to −273.15°C or −459.67°F.

Kelvin uses the same step size as Celsius: a temperature change of 1 K is identical to a change of 1°C. The two scales differ only in offset:

• Water freezes at 273.15 K (0°C)
• Water boils at 373.15 K (100°C)
• Body temperature: ~310 K
• Room temperature: ~293-298 K

Used in: scientific work universally, colour temperature (warm white lights ~2700 K, daylight ~5500 K, blue sky ~10000 K), thermodynamic equations where absolute temperature matters (gas laws, Stefan-Boltzmann), and any context where ratios of temperatures need to make physical sense (a ratio of Celsius temperatures across 0°C is meaningless because Celsius has an arbitrary zero).

Since 2019, Kelvin is defined via the Boltzmann constant rather than via the triple point of water. The definition change preserved every practical value to within metrology precision; nothing on a thermometer reads differently because of it.