how to convert from celsius to fahrenheit in your head

_how to convert from celsius to fahrenheit in your head_

(there's probably better ways on the internet, but everyone always asks me how i do it, so here's my process...

celsius is 9/5 of farenheit plus 32 degrees.

most people will tell you to multiply the temperature in celsius by 9, then divide by 5, then add 32. that works in the winter when the temperatures are low, but when the temperature is say 17, it takes me longer to multiply 17 by 9 and divide it by 5 than my method which involves doubling...

9/5 is equal to 1.8 which is equal to doubling the temperature and subtracting 20%.

so if it's 12 degrees celsius 

that means  12 x 1.8 + 32 = Fahrenheit Temperature

to multiply 12 and 1.8 quickly I just double 12

12 + 12 = 24

then subtract 20% (which is conveniently 10% of the doubled number, so just move the decimal point over in 24 and round if u want)

24 - 2.4 = 21.6 ...or if you don't care to be so precise, 24 -2 = 22

then add 32

21.6 + 32 = 53.6, 22 + 32 = 54

And there you have it, 12 degrees celsius is 53.6 degrees fahrenheit.

here's one more example:
21 degrees celsius = 70 degrees fahrenheit
21 doubled = 42
subtract 4 (or 4.2 to be precise) = 38 (37.8)

add 32 = 70 (69.8)

and finialy in the winter, if the temperatures are low, maybe try the other method:
5 degrees celsius = 41 degrees fahrenheit
5 times 9 = 45
45 divided by 5 = 9
9 plus 32 = 41