Absolute zero is exactly -273.15 degrees Celsius.

The bottom of the thermodynamic scale corresponds exactly to −273.15 degrees Celsius.

Kelvin and Celsius temperature increments have the same size; their zero points differ.

Absolute zero is the lower limit of ordinary thermodynamic temperature, not a claim that quantum systems have no energy.

Celsius temperatures are convenient for weather and household heating, but their zero is not the absence of thermal energy. Kelvin provides an absolute thermodynamic scale. The conversion between the two is an exact offset: add 273.15 to a Celsius reading to obtain kelvin. Thus a room at twenty degrees Celsius is at 293.15 kelvin. A temperature difference of twenty degrees Celsius, however, is simply twenty kelvin; adding the offset to an interval would be a conversion error.

The modern kelvin is defined through a fixed numerical value of the Boltzmann constant, which relates temperature to energy scales. Historically, temperature standards relied on reproducible material phenomena, including the triple point of water. Moving to a constant-based definition did not alter the Celsius offset or make water's freezing behavior independent of pressure and impurities. Water remains useful for practical calibration, but no particular sample now has to serve as the ultimate definition of thermodynamic temperature.

In a classical ideal gas, temperature is proportional to the average translational kinetic energy of the particles. Extending that picture suggests particles becoming motionless at zero kelvin. Quantum mechanics requires more care. A system can retain ground-state, or zero-point, energy even when it has no thermal excitations. The uncertainty principle prevents treating every particle as having both an exactly fixed location and exactly zero momentum. Absolute zero therefore does not imply that every form of motion or energy disappears.

Cooling methods exploit different physical mechanisms at different temperature ranges. Evaporation preferentially removes more energetic particles; laser cooling changes atomic motion through repeated interactions with light; magnetic and dilution techniques reach other ultracold regimes. Each method has limitations, and isolating a sample from unwanted heat becomes increasingly demanding. The unattainability formulation of the third law says absolute zero cannot be reached by a finite cooling process. Experiments can get extraordinarily close without demonstrating that the exact thermodynamic limit has been attained.

Reports of “negative absolute temperature” do not describe matter colder than zero kelvin in the usual sense. They concern specially prepared systems with bounded energy states and an inverted population, where the thermodynamic temperature parameter can be negative. Such systems can transfer energy to positive-temperature ones and are not an extension of an ordinary cooling sequence below absolute zero. For ordinary gases, liquids, and solids in everyday thermal equilibrium, zero kelvin remains the relevant lower limit. Distinguishing energy, temperature, and measurement scale prevents the familiar minus-273.15 figure from acquiring claims it does not support.

The unit symbol is K, without a degree sign. Writing “degrees kelvin” is obsolete terminology, whereas degrees Celsius remains correct. That notation reflects different scale conventions, not a difference in the size of their temperature intervals.

The exact Celsius-to-kelvin offset is a relation between scales, not a recipe for cooling matter. Reaching a lower reading in a laboratory requires removing energy from particular degrees of freedom, and those methods become increasingly demanding as the target temperature approaches absolute zero.