Conversions between Celsius and Kelvin scales can be done with simple addition an subtraction. Kelvin temperatures have much in common with Celsius temperatures, only the Kelvin scale has the advantage of setting absolute zero -- the coldest temperature possible -- at 0 Kelvin, or 0K. Note that while Fahrenheit and Celsius use the degree symbol (°) to denote units, it is not used for Kelvin temperatures.
Convert Celsius to Kelvin
The Kelvin scale was created in the 19th century by the British physicist William Thompson, later Lord Kelvin. The Kelvin temperature scale measures temperature in the same increments as the Celsius scale -- a 1K change in temperature is also a 1°C change in temperature. However, Kelvin uses a different starting point for its scale than Celsius. Absolute zero, the coldest possible temperature, is at 0 Kelvin, which is -273.15° in the Celsius scale. This makes the Kelvin scale a useful tool for scientists who want to measure temperatures by their distance from absolute zero.
Because of this, you can convert any temperature in Celsius to Kelvin by adding 273.15 to it. A warm summer day might have a temperature of 30°C. To convert it to Kelvin, add 273.15 to 30:
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30 + 273.15 = 303.15K
The warm day has a temperature of 303.15 Kelvin.
Convert Kelvin to Celsius
Since the a Kelvin temperature is 273.15 degrees higher than a Celsius temperature, you can convert a Kelvin temperature to Celsius through subtraction. The element mercury has a melting point of 234.31K; this means that at any temperature above 234.31K, mercury is a liquid. If you want to find this temperature in Celsius, simply subtract 273.15 from 234.31:
234.32 - 273.15 = -38.83 °C
So the boiling point of mercury is at -38.83 degrees Celsius.
Convert Fahrenheit to Kelvin
The conversion of a temperature from Fahrenheit to Kelvin is a little more complicated. The relationship between a Fahrenheit temperature (°F) and a Kelvin temperature (K) is given by the following equation:
K = (°F - 32)/1.8 + 273.15
The first part of this equation, (°F - 32)/1.8, converts the Fahrenheit temperature into Celsius. The second part converts Celsius to Fahrenheit by adding 273.15.
Say you've taken your temperature and found it's 99.5 °F. To convert it into Kelvin, you first subtract 32 from 99.5, getting 67.5. Then divide by 1.8 to produce 37.5. This is the temperature in Celsius. Add 273.15 to 37.5 to get the Kelvin temperature:
37.5 + 273.15 = 310.65K
So 99.5 degrees Fahrenheit is 310.65 Kelvin.
Convert Kelvin to Fahrenheit
You can also do this equation in reverse to convert from Kelvin to Fahrenheit. First, subtract 273.15 from the Kelvin temperature. Then multiply by 1.8. Finally, add 32. The equation for converting Kelvin to Fahrenheit looks like this:
°F = 1.8(K - 273.15) + 32
Oxygen gas condenses --turns from gas to liquid -- at a temperature of 90.188K. To find this temperature in Fahrenheit, first subtract 273.15, getting -182.962. This is the temperature in Celsius. Multiply -182.962 by 1.8, to get -329.3316. Finally, add 32:
-329.3316 + 32 = -297.3316 °F
So oxygen condenses at -297.3316 degrees Fahrenheit.