Converting Fahrenheit and Celsius

Updated January 25, 2022 | Infoplease Staff

Two very different scales

There are two different systems of temperature measurement. The first is the older Fahrenheit scale. The second is the younger and more popular Celsius scale. This is an article pretty much just for Americans or people traveling to the United States--America is one of very, very few countries that use the Fahrenheit scale anymore. Belize and the Cayman Islands also use Fahrenheit.

But, since they're both measuring temperature units, which are real things we can observe in the world, we can convert Celsius to Fahrenheit and vice versa. Temperature conversion is just a matter of changing our ratios using these conversion formulas.

Converting

For Celsius to Fahrenheit conversion, multiply the temperature by 9/5 and then add 32

For Fahrenheit to Celsius conversion, subtract 32 and multiply the remainder by 5/9.

Here are some important figures:

History

So how did we come up with the degrees Fahrenheit and degrees Celsius? Those have different answers, in that the Fahrenheit numbers are a bit off-kilter.

Physicist Daniel Gabriel Fahrenheit, who created the scale a couple hundred years ago, allegedly graded temperature from the freezing point of brine (water, salt, and ice) to around the average human body temperature. These are touchstones that one might find in nature, so it makes a degree of sense. But, we get odd numbers like water freezing at 32 degrees and boiling at 212 degrees, humans running at about 98 degrees, and room temperature being around 70 degrees.

Astronomer Anders Celsius made a system that's a bit more scientifically focused. It scales 100 degrees (making it "centigrade") between the freezing point of water and the boiling point of water at sea level. That is to say that water freezes at 0 and boils at 100. Celsius is officially used in the international system of units.

Kelvin Scale

Degrees Celsius (°C) and kelvins (K) have the same magnitude. The only difference between the scales is their starting points: 0 K is "absolute zero," while 0°C is the freezing point of water. One can convert degrees Celsius to kelvins by adding 273.15; thus, the boiling point of water, 100°C, is 373.15 K.

The Fahrenheit equivalent is the Rankine temperature scale.