The US is one of the few countries where the temperature measurements are in Fahrenheit. Elsewhere, Celsius is the norm. Usually, when traveling, I want an approximate mapping. Thankfully, it is linear and easy to memorize.

To convert temperature in ºC to ºF – double and add 30
To convert temperature in ºF to ºC – subtract 30 and half

Both of these are approximations for fast calculation

ºC ºF
0 30
10 50
20 70
30 90
40 105
50 120
The mapping is slightly inaccurate for ease of memorization.