[jiananli_]

A much faster approximation that is not too far off over the range of everyday non-extreme temperatures:

C → F: times 2 then plus 30

F → C: minus 30 then divide by 2

[bscphil]

Both of these are in the article, under "crude approximation". The criticism made in the article was that it can be off by ~3 degrees Fahrenheit at normal temperatures.

[thedanbob]

This is what I use. I'm not quick with arithmetic in my head so if I want anything more accurate it's faster to just grab my phone.

[thisacctforreal]

FWIW iOS Search (swipe down from the middle of the Home screen) can do conversions too.

https://i.imgur.com/16AAqDG.png