This is a temperature scale that physicists, astronomers, chemists and others in the physical sciences often use. Each unit kelvin is equal to the same change in temperature as a degree Celsius. But while Celsius places zero at the freezing point of water, kelvin places zero at absolute zero. This is the lowest possible temperature, where molecules can’t move. How cold is it? Zero kelvin is equal to −273.15° Celsius or −459.67° Fahrenheit.
The kelvin scale is named after the physicist William Thompson, the first Baron Kelvin. In the mid-1800s, he was the first person to correctly figure out just how cold absolute zero was.
The symbol for kelvin is “K,” and it does not use the term “degrees.” So if it’s 18 °C outside, it’s also 64 °F and 291 K.