Converting Temperatures

Converting temperatures between different scales. An article that will give you tips and an easy formula.

Most of the world uses the metric system to measure things including temperature. However, people in the United States still generally use the Fahrenheit temperature scale. Sure, most bank and other public thermometers display both Fahrenheit and the metric Celsius temperature scale, but very few people actually pay attention to anything but Fahrenheit.

This was brought home to me a few months ago when I was sitting at a conference with international attendees. It was winter, and quite cold here in Boston. Another attendee from Thailand was telling us how the Thai people were literally freezing to death because the temperature was nearing 20 degrees. I am quite used to interacting with people who think metric, and just mentally calculated the correct Fahrenheit temperature. I was amazed that people would freeze to death at around 70 degrees Fahrenheit and expressed that surprise. The person sitting next to just shrugged and said 20 degrees is cold.

Knowing metric measurements is more and more important even for Americans who still insist on using non-metric measurements in our every day lives. More and more of us email or otherwise interact with people from other countries and cultures. Both sides need a common ground for communications.



Converting between Fahrenheit and Celsius temperatures is quite easy. In these equations, F stands for a Fahrenheit temperature and C for a Celsius temperature:

F = C* 9/5 + 32 or, inversely, C = (F-32)*5/9

So to convert 20C to Fahrenheit, do the following:

F = 20*9/5+32 = 180/5+32 = 36+32 = 68 degrees

Similarly to convert 60F to Celsius, do the following:

C = (60-32)*5/9 = 28*5/9 = 140/9 = 15.56 degrees

If you just need an approximate idea of the value, you can estimate the conversion by keeping a mental list of a few corresponding values. Here are some baseline temperatures in both scales:

-40C = -40F

-18C = 0F

-10C = 14F

0C = 32F (freezing point of water)

10C = 50F

20C = 68F

30C = 86F

37C = 98.6F (human body temperature)

100C = 212F (boiling point of water)

Obviously, if you are interested in discussing the weather you don't need to know the boiling point of water. If someone starts talking about a mild 25 degrees, you know they are using the Celsius scale and that the temperature is between 50 and 68 degrees Fahrenheit just by knowing a few baseline temperature points.

In addition to the commonly used Celsius and Fahrenheit scales, a third temperature scale is used by scientists. Called the Kelvin and named after Lord Kelvin, it is the Celsius scale offset so that absolute zero has a value of 0. It is very easy to convert between Celsius and Kelvin:

K = C + 273.15 or C = K - 273.15

So absolute zero is 0K or -273.15C.

Similarly, using the formulae for converting between Celsius and Fahrenheit and between Celsius and Kelvin listed above, you can derive formulae for converting between Kelvin and Fahrenheit:

F=C*9/5+32=(K-273.15)*9/5+32=9/5K - 459.67

or

K=C+273.15=(F-32)*5/9+273.15=5/9*F+256.48

So absolute zero is 0K or -459.67F.

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