Here's an easy way for disadvantaged yanks to learn Celsius:
40C = 104F perfect hot tub temp
30C = 86F hot day
20C = 68F nice cool day
10C = 50F chilly day
0C = 32F freezing
Commit these to memory, then it's exactly 9F for every 5C in between. (or about 2:1)
[da fak with the downvotes? Just refuse to learn?]