100°C=373.15 K

To convert 100°C to Kelvin, add 273.15: 100 + 273.15 = 373.15 K. The offset of 273.15 represents the gap between the Celsius zero point (water's freezing point at standard pressure) and absolute zero (0 K), the theoretical minimum temperature at which all molecular thermal motion ceases. Since each Kelvin degree is exactly the same size as each Celsius degree, the conversion is purely additive — no scaling factor is needed.

The Kelvin scale is the SI base unit for thermodynamic temperature and has a fundamental property that Celsius and Fahrenheit lack: it starts at absolute zero (0 K = −273.15°C), meaning Kelvin values are never negative. This makes Kelvin ideal for scientific and engineering calculations, particularly those involving ratios or proportional reasoning. For instance, the ideal gas law (PV = nRT) and blackbody radiation formulas require temperatures in Kelvin because doubling the Kelvin value truly doubles the thermal energy — a relationship that does not hold for Celsius or Fahrenheit.

Kelvin appears throughout thermodynamics, astrophysics, chemistry, and materials science. The surface temperature of the Sun is approximately 5,778 K. Liquid nitrogen boils at 77 K (−196°C). Superconducting materials must be cooled to a few Kelvin. In these contexts, 100°C corresponds to 373.15 K — a value that scientists and engineers work with directly when applying temperature-dependent formulas or comparing readings across instrumentation that reports in Kelvin. Laboratory equipment such as spectrometers, cryostats, and calorimeters often displays readings in Kelvin.