Celsius (°C) is the most widely used temperature scale worldwide, where 0°C marks the freezing point of water and 100°C its boiling point at standard atmospheric pressure. It serves as the default temperature unit in nearly every country except the United States, appearing in weather forecasts, cooking instructions, and scientific literature. To convert Celsius to Fahrenheit, multiply by 9/5 and add 32.

Kelvin (K) is the base unit of thermodynamic temperature in the International System of Units, where 0 K represents absolute zero, the theoretical point of zero thermal energy. Scientists use Kelvin in physics, chemistry, and astronomy because it starts at true zero with no negative values, making calculations straightforward. Zero degrees Celsius equals 273.15 Kelvin, and the two scales share the same size of degree increment.