What is a Kelvin?

The Kelvin is the SI base unit of thermodynamic temperature, named after British physicist William Thomson, 1st Baron Kelvin (1824–1907), who proposed an absolute temperature scale in 1848. Kelvin starts at absolute zero — the theoretical lowest temperature where all classical molecular motion stops — at exactly 0 K. The Kelvin uses the same degree size as Celsius: 0°C = 273.15 K, water boils at 373.15 K. Note that Kelvin temperatures are not preceded by a degree sign (300 K, not 300°K). The Kelvin is essential in scientific work — gas laws, blackbody radiation, cryogenics, and astrophysics all use absolute temperature. Since 2019, the Kelvin has been defined by fixing the numerical value of the Boltzmann constant (k = 1.380649 × 10⁻²³ J/K). The Kelvin relates to Celsius by addition (K = °C + 273.15) and is universally used in physics and chemistry.

Visual reference for how the kelvin relates to other temperature units. Each row links to the full converter for that pair.

• Scientific and engineering thermodynamics
• Astronomy and astrophysics temperature reporting
• Colour temperature of light sources (e.g., 6500 K daylight)

• ✓K = °C + 273.15. Identical degree size; only the zero point differs.
• ✓There are no "degrees Kelvin" — just "kelvins". The symbol is K, not °K.
• ✓Colour temperature: 3000 K warm/yellow; 5000 K daylight; 6500+ K cool/blue.

• ✗Writing °K — incorrect. Kelvin uses no degree symbol.
• ✗Trying to use negative Kelvin values — impossible physically.
• ✗Confusing "kelvin" (unit) with "Kelvin" (the scientist William Thomson).