Paul Tomblin wrote:
In a previous article, Ron Natalie said:

Paul Tomblin wrote:

Add 40. Multiply by 9/5 or 5/9 depending on whether you're going F-C or
C-F. Subtract 40. Works both ways, exactly.



40? Where does 40 come from?


Try it, it works. -40 is where the two values are equal, so essentially
you're moving the graph "F versus C" so that it goes through the origin,
applying the slope of the line, and then shifting it down again.

-40 + 40 = 0
0 * 5/9 = 0
0 - 40 = -40 - -40 F = -40 C

0 + 40 = 40
40 * 5/9 = 22.22
22.222 - 40 = -17.7777 - 0 F = -17.8 C

212 + 40 = 252
252 * 5/9 = 140
140 - 40 = 100 - 212 F = 100 C

And so on.

Oh I missed that you were always adding and subtracting 40 (it didn't
follow the "depending on weather..." Of course if you use 32 as your
contant (the difference between 0C and 0F), you don't have to apply the
offset twice.