Celsius and Kelvin are two units of measuring temperature. In the scientific world, these two means of calibration are very important. Indeed, they possess peculiar similarities which may be the reason for the confusion between them. Nevertheless, they also have very distinct characteristics that are used in practical applications in many fields of study or industry.
Foremost, the Celsius system was devised by Anders Celsius back in 1742. Until 1954, Celsius was only described in terms of the boiling (100^0C) and freezing (0^0C) points of water (under standard conditions of atmospheric pressure). This was the case because of its convenience in calibrating instruments for temperature gauging like the common thermometer, thermocouple ,and the Boyle’s apparatus.
But post 1954, Celsius has been given a new definition by including the variables of water’s triple point and the concept of absolute zero. With regard to absolute zero, it is actually precisely at -273.15^0C while the triple point is at 0.01^0C. Ironically, the first one is still the classic definition of Celsius being taught in many learning institutions today.
By contrast, the Kelvin was coined in honor of Sir William Thomson the Baron of Kelvin. He was the one who proposed that there should be another system with the same size of unit as Celsius but different in describing the 0 degree as the absolute zero (not under standard atmospheric pressures or where matter is showing no entropy – an unattainable temperature in reality). With this, Kelvin is an absolute temperature measurement scale.
This is also probably the reason why it can easily be incorporated to any type of mathematical equation having variables of temperature. In the case of Celsius, you first need to convert it to Kelvin for it to be applicable. With regard to conversion, it’s quite easy, actually. Just add 273.15 to your Celsius value if you want to convert it to Kelvin (K = ^0C + 273.15) whereas subtract the same constant from the given value of Kelvin to convert it to Celsius (^0C = K − 273.15).
The Kelvin scale uses absolute zero and triple point in its definition which is why they have similar unit measures with Celsius. With this it can simply be stated that -273.15^0C is equal to 0 K while 0^0C is equal to -273.15 K. if you carefully look into their relationship, you’ll realize that the lowest point attainable in a Kelvin scale will be zero as there will be no negative temperatures in that type of system. With this, 0 Kelvin really is zero and there is no colder temperature below that unlike in Celsius.