[u801e]

The way I convert is to remember that 32°F is 0°C and then count by "nines" for Fahrenheit to get the desired temperature in Celsius.

  32°F = 0°C
  41°F = 5°C
  50°F = 10°C
  59°F = 15°C
  68°F = 20°C
  77°F = 25°C
  86°F = 30°C
  95°F = 35°C

It's also pretty easy to remember that 10°C = 50°F and 35°C = 95°F if you want to go forward or back from there with the same counting method.

[tomger]

For weather purposes the approximation seems accurate enough. Here’s a table listing the differences: http://tomgermeau.com/tools/FC/

[frutiger]

This is not an approximation. The conversion formula is:

C = 5/9 * (F - 32)

So GP's table is exact.

[tomger]

The article refers to C = 2 * (F - 30) as an approximation

[frutiger]

Ah, I had mistaken your reply, and assumed it was to /u/u801e and not the linked article.