As someone who has to work with both temperature systems, I use this for a quick and easy way to get a number close enough to understand.
For C to F "double it and add 30" so 20 C = (2x20) + 30 = 70F which is close to the real value of 68F to know it's a nice day.
F to C "subtract 30 and halve it" so 100F = (100-30)/2 = 35C which is close to the actual value of 37.8C and getting pretty warm.
It's not perfect, but it is quick and easy enough to do in your head.
Another trick? Remember that:
0C is 32F
16C is 61F, and
28C is 82F.
If the number you want is close to those 3 numbers then just treat 2F = 1C and work it out from there. What is 20C? 16C + 4 = 61F + 8 = 69F which is close enough to 68F for me. What is 26F? 32-26 = 6F or 3C so it's 3C below freezing or -3C.
(Side note, -40C is the same as -40F but by then who cares, that's just seriously cold everywhere...)
Multiplying by 5/9? Not exactly "easy" though...
As someone who has to work with both temperature systems, I use this for a quick and easy way to get a number close enough to understand.
For C to F "double it and add 30" so 20 C = (2x20) + 30 = 70F which is close to the real value of 68F to know it's a nice day.
F to C "subtract 30 and halve it" so 100F = (100-30)/2 = 35C which is close to the actual value of 37.8C and getting pretty warm.
It's not perfect, but it is quick and easy enough to do in your head.
Another trick? Remember that:
0C is 32F
16C is 61F, and
28C is 82F.
If the number you want is close to those 3 numbers then just treat 2F = 1C and work it out from there. What is 20C? 16C + 4 = 61F + 8 = 69F which is close enough to 68F for me. What is 26F? 32-26 = 6F or 3C so it's 3C below freezing or -3C.
(Side note, -40C is the same as -40F but by then who cares, that's just seriously cold everywhere...)