How can you directly convert fahrenheit to celsius if they were created with different methods of measurement?
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All you need to know to convert them is the following two things:
(A) An assumption that both use a linear scale. (that the 1 degree difference between, say, 2 degrees and 3 degrees is the same size difference as the 1 degree difference between 50 and 51 degrees, or 90 and 91 degrees, etc. That one degree of difference means the same thing at all places along the scale, for both scales.)
(B) You also need two sample temperatures that you know are the same phenomenon for both scales so you know they equate. That can fix the two scales relative to each other. For this you can use water’s freezing and boiling points.
Whatever Fahrenheit claims is water’s boiling point in its scale is the same physical temperature as what Celsius claims water’s boiling point is.
Whatever Fahrenheit claims is water’s freezing point in its scale is the same physical temperature as what Celsius claims water’s freezing point is.
That tells you that what Fahrenheit claims should be called 32 is the same physical thing as what Celsius claims should be called 0. And what Fahrenheit claims should be called 212 is the same physical thing as what Celsius claims should be called 100.
That is how the conversion formula is made.
The span between freezing and boiling takes Fahrenheit takes 180 degrees to cover, while Celsius covers that same range in 100 degrees.
That tells you that one Celsius degree is 1.8 times as “big” as one Fahrenheit degree. And you know the zero point of Celsius is the place where Fahrenheit is 32.
Putting those two together gives you:
F = 1.8 C + 32
(Or, the other way:)
C = (F – 32) / 1.8
(For either of the above, some places show the 1.8 part of the formula as 9/5. It’s the same thing.)
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