As the Celsius temperature scale gains more acceptance in society, it becomes more important in daily life to be able to make the conversion from Fahrenheit to Celsius and back. Celsius may sometimes be called Centigrade by old school science students. The more commonly accepted name is Celsius.
The freezing and boiling point of water at sea level is important to both scales.
Both scales use the freezing and boiling point of water at sea level as benchmarks for their temperature calibrations. Fahrenheit sets the boiling point of water at 212 degrees and the freezing point at 32 degrees. Celsius makes it a little simpler by putting 100 degrees as the boiling point and 0 degrees as freezing.
It is the relationship regarding these benchmarks that allows the conversion of temperatures from one to the other.
To find out the relationship between these two popular ways to measure temperatures, start with the freezing point of water on the Fahrenheit scale. Subtract the freezing point of water on the Celsius scale. The difference is, of course, 32 degrees. However, it is evident that this is not the only conversion necessary because when you subtract 32 degrees from 212 you get 180 not 100. This points to the need of a two step process.
Use the difference at the freezing point of water to determine the other part of the conversion formula.
Once you have subtracted the 32 degrees from the boiling point of water and arrived at 180 degrees, you must now compare the boiling point of water on the Celsius scale at 100 degrees. If you divide the Celsius scale into the 180 degrees, you get 1.8 or 9/5. You can use either one for the conversion.
Armed with these two pieces of information, a conversion from Fahrenheit to Celsius can now be accomplished.
Starting with 212 degrees, subtract 32 degrees. This brings you to 180 degrees. Divide this number by 1.8 or by 9/5 whichever is easiest for you. Do not forget to invert the fraction and multiply to do the division by 9/5. Your answer will be 100 degrees. So, 212 degrees Fahrenheit equals 100 degrees Celsius.
To see a number that is not so obvious, use 113 degrees Fahrenheit.
This would be a typical day in most southern deserts in the middle of summer. Subtract the 32 from 113. This will give you 81. Now, divide 81 by 9/5. You have turned a 113 degree sweltering day into a 45 degree Celsius temperature. It is just as hot, but sounds much cooler to Fahrenheit people. For a temperature nearer to home, consider 77 degrees Fahrenheit. Subtract the 32 degrees and get 45 degrees. Do the division by 9/5. 77 degrees Fahrenheit has become 25 degrees Celsius.
Both scales register the same temperature at only one point.
At minus 40 degrees, Celsius and Fahrenheit thermometers will record the same temperature. To prove this, start with the -40 degrees. Subtract 32 degrees. This will give you -72 degrees. Divide this by 9/5. Presto! You have arrived back where you started at -40 degrees, but this time it is Celsius. It might still be Fahrenheit, but it does not matter they are both the same for one degree and diverge again.