Water freezes at 273 Kelvin. Kelvin is a temperature scale named after William Thomson, also called Lord Kelvin. The Kelvin scale starts at zero, the lowest possible temperature.
Zero Kelvin is also called absolute zero. It is the temperature at which molecular energy is at a minimum. To convert from Kelvin to Celsius, 273.15 is added to the Kelvin temperature. Pure water freezes at 273.15 Kelvin and boils at 373.15 Kelvin.
Kelvin is commonly used in physics, chemistry and astronomy. The Kelvin scale is easier to use in formulas and calculations than Celsius because it has no negative values. Because of this, the Kelvin scale is the international standard for temperature measurements.