In the formulas below, / represents division and * represents multiplication.
Tc = (5/9)*(Tf-32); Tc = temperature in degrees Celsius, Tf = temperature in degrees Fahrenheit
For example, suppose you have a Fahrenheit temperature of 98.6 degrees and you wanted to convert it into degrees on the Celsius scale. Using the above formula, you would first subtract 32 from the Fahrenheit temperature and get 66.6 as a result. Then you multiply 66.6 by five-ninths and get the converted value of 37 degrees Celsius.
Below is the formula to convert a Celsius scale temperature into degrees on the Fahrenheit scale.
Tf = (9/5)*Tc+32; Tc = temperature in degrees Celsius, Tf = temperature in degrees Fahrenheit
Assume that you have a Celsius scale temperature of 100 degrees and you wish to convert it into degrees on the Fahrenheit scale. Using the stated formula, you first multiply the Celsius scale temperature reading by nine-fifths and get a result of 180. Then add 32 to 180 and get the final converted result of 212 degrees on the Fahrenheit scale.
Below is another accepted conversion method that works just as well and perhaps might be easier to remember. No matter which direction you want to covert, Fahrenheit to Celsius or Celsius to Fahrenheit, always first add 40 to the number. Next, multiply by 5/9 or 9/5 just like the first method. Then, always subtract out the 40 you just added to yield the final result. To remember whether to use 5/9 or 9/5 when converting from Fahrenheit to Celsius or Celsius to Fahrenheit, just simply remember, F (for Fahrenheit) begins with the same letter as Fraction. 5/9 is always a Fraction; while 9/5 is also a fraction, in this form, it is Clearly a whole number plus a fraction (1 and 4/5). Thus, if you want to convert Fahrenheit (F) to Celsius (C), then use the Fraction 5/9; Celsius (C) to Fahrenheit (F), use the other, 9/5, which is Clearly not just a fraction.
For an example of this method, we'll use the values we used in the initial example, 98.6 F and 37 C, which are equal.
To convert from F to C, try these calculations manually.
98.6 + 40 = 138.6, and 138.6 * 5/9 = 77. For the final calculation, remove the 40. 77 - 40 = 37
To convert from C to F, try these calculations manually.
37 + 40 = 77, and 77 * 9/5 = 138.6. For the final calculation, remove the 40. 138.6 - 40 = 98.6
In summary, add 40, (F to C) multiply by Fraction...(C to F) multiply by the other, subtract 40.
The Celsius temperature scale is still sometimes referred to as the "centigrade" scale. Centigrade means "consisting of or divided into 100 degrees;" the Celsius scale, devised by Swedish Astronomer Andres Celsius (1701-1744) for scientific purposes, has 100 degrees between the freezing point (0 C) and boiling point (100 C) of pure water at sea level air pressure. The term Celsius was adopted in 1948 by an international conference on weights and measures.
I have never been able to memorize these formulas but have worked out a way to reconstruct them. I only need to remember three things:
1. zero Celsius is 32 Fahrenheit (the melting point of water)
2. Fahrenheit number rise and fall a lot faster than Celsius
3. 5/9ths or 9/5ths is involved somewhere in the formula.
Then the question is, how to combine these three facts?
I also know intuitively that if you're going to start multiplying things you'd better have a stable zero-point. For example, at sea level, the top of my head is at elevation six feet. If you double my elevation by zapping me with a grow-raygun I become 12 feet tall. That works fine. But when I'm standing 32 feet above sea level (and my head is at 38 feet above sea level) if you double my head's elevation (to 76 feet) you have to make me 44 feet tall (feet still at 32 feet)! That's absurd! What's wrong? mismatched zero points.
Okay. I have a Celsius number. It has a good zero point that I know corresponds to something in Fahrenheit. I know (fact 2) that Fahrenheit grows faster, so that means I need to use 9/5 (not 5/9) and multiply the C number by 9/5. Then (with rule 1) I need to move the F number up by 32 to give compensate for the mismatched zero points.
The other way around, starting with F: Fahrenheit has its "feet" at 32, so I need to take it down to match Celsius' zero point, so I subtract 32. Then, knowing rule 2, I know 5/9 (not 9/5) needs to be used, so I multiply by 5/9. Done.
I check my reconstruction by also knowing that 100C = 212F (boiling point of water) just to make sure I got it right.