What is the exact value of absolute zero in degrees Celsius?

Absolute zero is exactly −273.15 °C on the Celsius scale.

Absolute zero is defined as 0 kelvins, the lowest limit of thermodynamic temperature. The Celsius and kelvin scales have equal-sized degrees but different zero points: 0 °C corresponds to 273.15 K. Therefore, the conversion is °C = K − 273.15, making 0 K exactly −273.15 °C.

The value is a defined scale relationship, not merely a modern experimental estimate. Early gas-thermometry experiments approached the value by extrapolating how gas pressure or volume changes with temperature. Those experiments helped reveal that temperature has a lower physical limit.

Absolute zero cannot be reached by a finite physical process, according to the third law of thermodynamics, although laboratories can cool systems extremely close to it. The nearby values −273 °C and −273.0 °C are rounded approximations, while −273.16 °C is incorrect for absolute zero.