If you're from the United States, you're probably accustomed to describing temperature by Fahrenheit. A very hot summer day is 100 degrees, a pleasant spring morning is 50 degrees, and 0 degrees is ungodly cold. If you're from almost anywhere else, you likely favor Celsius, where 0 degrees is only pretty cold, 50 degrees is unspeakably hot, and 100 degrees happens on a stove, not outside.
What's the difference between Celsius and Fahrenheit? It's really a quirk of history and a simple mathematical conversion. Here follows the easiest way to convert Fahrenheit to Celsius, as well as the reverse calculation.
Let's cut to the chase. Here are the two equations you need:
In other words, if you'd like to convert a temperature reading in Fahrenheit to Celsius:
Using this calculation, we determine that 100 degrees Fahrenheit is equivalent to 37.78 degrees Celsius. The reverse conversion -- from Celsius to Fahrenheit -- is similarly straightforward:
Thus, 100 degrees Celsius is equal to 212 degrees Fahrenheit.
At -40 (i.e., "minus 40"), nothing! If you were wondering if there is a temperature where Fahrenheit and Celsius are the same, it's at 40 below zero. At all other temperatures, the difference is history.
The Fahrenheit scale was created by Daniel Gabriel Fahrenheit in 1724. Fahrenheit invented his scale for use with mercury thermometers, which he also invented. The Fahrenheit scale is a bit older than the Celsius scale.
The Celsius scale was created by Anders Celsius in 1745, a couple decades after Fahrenheit. Curiously, Celsius created the reverse of the modern scale, treating 0 as the boiling point of water and 100 as the freezing point of water. Other scientists, notably Carl Linnaeus, flipped it later that year, making 0 the freezing point and 100 the boiling point of water.
Since it was originally built as a scale between 0 and 100, Celsius is also called centigrade. The word "centigrade" literally describes something that consists of or is divided into 100 degrees. That said, Celsius has been the preferred nomenclature since 1948.
Globally, the standard scale for everyday use is Celsius. Only the United States, islands freely associated with the US (Palau, the Marshall Islands and Micronesia), the Bahamas, the Caymans, and Liberia use Fahrenheit as their primary temperature measurement. Everybody else goes with Celsius.
You may have heard of temperature being measured according to kelvin. Indeed, if you're involved in the sciences, you probably use kelvin. Kelvin is a bit different than Fahrenheit and Celsius. Celsius and Fahrenheit are both temperature scales, measuring temperatures in degrees.
Named after Lord Kelvin, a kelvin is a standard scientific unit, like a kilogram for weight or a meter for distance. The proper way to use the word, then, is not "100 degrees Kelvin" but "100 kelvin." (Note the lower case "k" in kelvin.)
1 kelvin equals 1/273.16th of the triple point of water: that is, the point at which liquid water, water vapor and solid ice can coexist. In other words, 273.16 kelvin is the exact triple point of water.
The importance of kelvin is that it measures absolute temperature: absolute zero is 0 kelvin, the temperature at which all atomic motion (which creates heat) stops. There is no thermal energy whatsoever at absolute zero.
Kelvin is primarily of value to scientists, since it simplifies calculation at extreme temperatures. For your convenience, the conversion equations follow.
That is, you convert Celsius to Kelvin by simply adding 273.15. If the outside temperature is 10 degrees Celsius, then it is 283.15 kelvin. That's easy enough.
Fahrenheit is a bit trickier. Take your degrees Fahrenheit, subtract 32, divide by 1.8, and finally add 273.15. Here's a quick example:
Thus, 100 degrees Fahrenheit is equal to 310.93 kelvin.
Temperature is important! Measuring heat accurately matters in every field of human endeavor, from engineering to medicine. For more on how to manage your temperature, take a look at this automatic Celsius to Fahrenheit converter. It'll make short work of all your temperature conversions.