Celsius (abbreviation: °C) is a temperature scale with the freezing point of water and the triple point of water being defined as 0 degrees Celsius (or °C), and the boiling point of water being defined as 100 degrees Celsius (or °C). This scale is widely used in science, particularly in countries where the metric system is widely used, although it is officially limited to temperatures between −50° and 100°. It is also the basis of a number of other scales, including the APH (Atmospheric Pressure-based Humidity) scale and the ANSI standard for reporting weather temperatures. Celsius is named after the Swedish astronomer Anders Celsius. The degree Celsius (°C) can refer to a specific temperature on the Celsius scale as well as a unit to indicate a temperature interval, a difference between two temperatures or an uncertainty. For example, it may be used to describe the temperature at which water boils, or that at which the pressure of a given gas is equal to 1 atmosphere (atm).

The kelvin (symbol: K) is a unit of measurement for temperature. It is named after the London engineer and physicist William Thomson, who was elevated to the peerage as Lord Kelvin in 1892, four years after his death. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all thermal motion ceases in the classical description of thermodynamics. The kelvin unit is equal to one quadrant of the thermodynamic temperature scale (at least in theory), which ranges from zero to a maximum value of plus and minus 273.15 degrees. The Kelvin scale is widely used in science and technology. The Kelvin temperature unit is used as a measure of absolute temperature. It's the equivalent of 0° Celsius and 273.15 K (degrees Kelvin) equals -273.15° Celsius, or 0° Kelvin is equal to the triple point of VSMOW (standard mean ocean water). There are several combinations and conversions of units that involve the Kelvin temperature. To convert between Fahrenheit and Celsius using kelvins, you must add 273.15 to a number in Fahrenheit to get its value in kelvins. For example, adding 273.15 to 212 degrees Fahrenheit will give you about 373.15 kelvins.