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To convert Celsius to Kelvin, add 273.15. So 0°C = 273.15 K (water freezing), 100°C = 373.15 K (water boiling), and −273.15°C = 0 K (absolute zero). The Kelvin scale has no negative values — 0 K is the coldest possible temperature in the universe.
Kelvin is the SI unit for thermodynamic temperature and is used exclusively in science. Chemistry, physics, and engineering calculations — particularly the ideal gas law (PV = nRT) — require temperature in Kelvin. Astronomers describe stellar temperatures in Kelvin: the Sun's surface is about 5,778 K.
Did you know? Absolute zero (0 K = −273.15°C) is the temperature at which particles have minimum possible energy. It has never been reached in practice, though laboratories have achieved temperatures within billionths of a degree above it. The Kelvin scale was proposed by Lord Kelvin (William Thomson) in 1848.
Physics and chemistry experiments require temperatures in Kelvin for gas law and thermodynamic equations. Material science describes phase transitions in Kelvin. Cryogenics and superconductor research work near absolute zero — liquid nitrogen at −196°C is 77 K. Climate science sometimes uses Kelvin for modeling.